Since I started playing League of Legends about six months ago, Irelia has been the only champion I bought directly when it was launched. I enjoyed playing her so much and played her in so many different ways I decided to pick my favourite way of playing her and sharing it with you(thanks DEWO, P.DiddyXD and many others for sharing your builds). This is my first build on Mobafire so I'm kind of a newby with the site's mechanics.

I do not play a lot of ranked games, because I get disturbed quite a lot and because of that I'm not able to always play a game untill it ends. I don't want to bother other people by leaving a ranked game. Because I don't play ranked, I'm not sure this build is any good in ranked games, but I found this build extremely effective in normal games.

Here we go!

Irelia fits a lot of roles quite well, if built well. She can be built in many ways that are all effective if played smart.
In my opinion, her ability to proc Sheen every time Sheen comes off cooldown is the reason why I think Irelia is the greatest champion ever. It doesn't matter if you use an AP or AS based build, a tanky or armor pen based build... if you use a Sheen, Triforce or Lich Bane, Irelia rocks your socks!

Marks:

Greater Mark of Desolation. Attacks procced by sheen do physical damage, you want Irelia to ignore that tiny bit of armour the enemy carry has.

Some people would use attack speed or even magic penetration marks on Irelia. The problem with Attack speed is that it doesn't help your sheen-based damage and it's only fifteen friggin' percent (just buy a friggin' dagger if you want it so badly). Picking magic pen marks is rediculous for three reasons:
1. They're only half as strong as armor pen marks.
2. Buying magic pen items is a lot cheaper than buying armor pen items.
3. THEY DON'T SUPPORT YOUR SHEEN PROCS!

Seals:

Greater Seal of Evasion. It's awesome in teamfights. You're going to get focussed a lot, cause you're Irelia and guys like to hit girls! So we pick dodge runes and Nimbleness to use that to your advantage, you dodge and you gain 33 movement speed, yay, kiting just got a lot easier!

Some other people use health or mana/5/lvl seals, or hell, even attack speed seals (lolwut? Secondary runes?). The nice thing about health runes is that they make you really beefy early game, but their effectiveness decreases as you level up. Mana/5/lvl seals are great if you're going to spam Q all the time to farm minions, but you really don't need your Q to do that properly. This build gives you enough CD reduction to use your ultimate to clear minion waves. I'm not even going to talk about attack speed seals, it's not worth my time.

Glyphs:

Greater Glyph of Scaling Magic Resist and Greater Glyph of Magic Resist. We all hate those annoying mages harassing you all the time, so we need magic resistance to decrease the damage done. You W will let you recover health faster than they can hurt you. I pick a mix of flat magic res and magic res/lvl glyphs to keep their effectiveness at a steady level throughout the game.

Other people would pick cooldown reduction or mana/5/lvl glyphs, or even more attack speed (herp-a-derp!). From my experience, Cooldown reduction is the only worthy substitute to magic resistance glyphs, for one reason. They help you get more Sheen procs! However, because smart people will harass you a lot and you're getting a Glacial Shroud in mid game, you really don't need them as much as magic res glyphs.

Quints:

Quintessence of Health. I know I wrote they lose their effectiveness in mid/late game (and that's true), but Irelia is a melee champion and melee champions need more health to make up for the damage they take while closing in on ranged champions. I pick HP quints instead of HP seals because HP quints have a much better 'scaling' than HP seals (26 vs. 5.35, five times more effective, 1.66/ times more effective if you take into account the number of quint- and seal slots).

If you think you're such a badass, you can take armor pen quints instead of HP quints, but we can't all be badasses, cause then nobody would be a badass.

Runes for dummies:

1. Always use primary runes. Runes are marked by the color of their 'aura', the color of the circle around the rune itself. If the color of the aura matches the color of the rune's slot, it's a primary rune. For example, the aura an armor penetration mark is red, the same color as a mark's slot, therefore, it's a primary rune. You can find the math behind primary and secondary runes here. Thank Searz for that.

2. When you're not able to buy any level three runes yet, buy a full level one rune book. Level one runes are extremely cheap, but they do have a noticable impact on your success in-game.

3. Don't use many different kinds of runes. You don't want your champions to be 1% better at everything, you want them to be 10% better at a few, more important things!

I think picking 21 points in any mastery tree is a waste of Irelia's potential do be very versatile. I like to spread my masteries to be able to do whatever my team needs. With these masteries, especially because of Nimbleness , you can kite and carry your way to victory more easily. Nimbleness is a mastery to make use of. You see an enemy farming happily in his lane. You jump him and his minions will start attacking you. You'll dodge a minion's attack and gain 33 free movement speed! This way, Vladimir can blood pool all he wants, he's going down.
Another mastery to abuse is Awareness . With this mastery, you will reach lvl 6 before your enemies will, allowing you to use your ultimate, while they can't. This usually helps me get a kill or two the moment I hit lvl 6.
I get Archaic Knowledge because it's a cheap way to get lots of magic pen. Even with the enemy having only 50 magic resistance, you get about equal the amount of magic pen as when getting nine magic pen marks.

The key to Irelia's damage output is combining smart skill usage with Sheen. This is the reason why so many players fail with Irelia, you have to be smart to dominate with her.

The first item I like to buy is a Doran's Shield, because there's one thing that Irelia lacks, and that's a skill which increases her survivability. The shield lets you take some more minion aggro, it lets you recuperate faster from repeated harassing and it just gives you an overall advantage, because Irelia's skills do quite a lot of damage by themselves.

Boots, boots, boots... every game is different. If you feel you need more DPS, get Ionian Boots of Lucidity or Sorcerer's Shoes. If you feel you need more defense, get Mercury's Treads. If your team doesn't have map awareness, get Mobility Boots to be able to come to their aid quickly (or buy them some wards, dont be stingy, you're Irelia)

Once you get Sheen on your first trip back, you can stop thinking about offensive items for a while. You want to tower dive! Getting armor is the way to do this, buy a cheap Chainmail for 700 gold and notice the difference! People will be scared of you - at least, they should be. It's nice to have the enemy feeling helpless, they will start flaming their teammates for not helping them get rid of you (YES!!)

During mid game, you'll want to upgrade your Sheen into a Trinity Force, because you still want to be able to take down those tanks who now start to beef up.
Also, I suggest you try upgrading that Chainmail into the Glacial Shroud. 15% cooldown reduction packs a lot of punch, especially when you rely on Sheen procs for your damage output. We can now move on to late game.

If your team is doing well, keep buying offensive items, I suggest Zeke's Harbinger when you got more than 2 physical attackers in your team. If not, get a Black Cleaver to have some Ashe-raping fun (pun intended). The newly improved Black Cleaver (improved because of faster armor eating) is great for irelia, because you max the armor penetration in one and a half second by using Q and two auto attacks.
If your team is not doing well, you should anticipate on that by buying items like Banshee's Veil and Guardian Angel, stuff that keeps people from wanting to kill you first.

Other items you want are Potions and Wards. Potions are incredibly useful to turn the tides in a teamfight, especially in mid game. I suggest getting either green or blue potions, or both. You should keep the map warded at all time and you're pretty much guaranteed a victory. Co-ordinate with your team to be sure you're not the only one buying wards.

I'm going to clarify something now. You can rely on Sheen for Irelia's damage output. On lvl 18, a Trinity Force will make her inflict 112*1.5=168 extra damage every two seconds (Sheen proc cooldown) and you can add her crit chance and true damage to that. So even when having only a Trinity Force as an offensive item, she does significant damage.

I like to take one point in Bladesurge for mobility and then spreading skill points over Hiten Style and Equilibrium Strike, getting a point in Transcendent Blades whenever I can.
The reason I ignore Q is because it doesn't add as much to a figh as W and E do. Q is nice for farming, but it eats mana and using R for clearing a minion wave is much better. R has a low cooldown anyway.
The reason I don't focus on maxing a certain skill ASAP is because that would hurt Irelia's potential. Maxing E leaves her without proper DPS and maxing W leaves her without proper crowd control.
If you're getting dominated in your lane, you should prioritize W over E, because with W, you can regain large amounts of health in a short amount of time. You will want at least one point in E to punish players trying to dive you. If someone dives you, let him attack you once. Then run back a bit to get him closer to your turret, then Q towards him, followed up by a fast E for a nice stun. Then attack him until he dies. Now hope he ragequits.

Bladesurge (Q)
During the laning phase, you should use this skill to last hit minions you can't last hit with a normal attack. When two minions have low health and you can't last hit both of them, kill one with a normal attack and quickly use Q to last hit the other one. When farming, make sure Q is never on cooldown, since it refunds 35 mana if you get a kill using Q.
To start a gank, use Q to initiate.
In teamfights, you can use Q to increase your mobility. For example, when the enemy team has a Nunu and he uses his ultimate Absolute Zero, use Q on an enemy who is outside of the ultimate's range. You can also use Q to dodge skillshots like Blitzcrank's grab Rocket Grab and Ashe's ultimate Enchanted Crystal Arrow.

Hiten Style (W)
In the laning phase, when you only have 1 or two points in this, it's only good for proccing Sheen and dealing a bit of bonus damage (which isn't bad). Only use this skill on enemy champions and for healing.
In teamfights, when this skill is at a higher level, you want to save this skill for taking out armor stackers like Malphite and Rammus. Although it's also nice to kill squishies faster, I feel it's better used against tanks, because killing squishies shouldn't be a problem anyway.

Equilibrium Strike (E)
In the laning phase, use this skill to turn fights for first blood in your favor. This skill is best saved for when you have lower health than the enemy, because then it will stun instead of slow. If you feel the health gap between you and your enemy is too large, feel free to use Q to dash towards him, use E so he gets stunned, then run back. Repeat if necessary. Equilibrium Strike is also great for stopping people from ganking you. Because of this skill, it's pretty safe for Irelia to scout and to facecheck bushes.
In teamfights, use this skill to disable the enemy carry as much as you can (if you didn't kill him already). Another, more defensive way to use this skill is to disable the enemy attacking your carry/supporter.
Because of Equilibrium Strike, Irelia is one of the few characters who is comfortable when having low health. It's a great way to turn 1 v 1 fights and tower dives around, in your favor.

Transcendent Blades (R)
Your ultimate can be used for many different purposes. Since it has a very low cooldown, I like to use it to clear minion waves fast.

Secondly, I use this skill to regain health even faster than Mundo does. Just stand in front of a minion wave and fire all charges at the minions. Because I focus on resistance instead of health in my item build, I'm able to restore more than half of Irelia's HP in a couple of seconds.

Another way is to kill low-health enemies who are trying to run away. R has a large range and it doesn't interrupt your movement, so you can use it while attacking someone else.

My favorite way of using Irelia's ultimate is to activate it and shooting a blade every two/three seconds to proc Sheen. This way of using her ultimate is the reason why even an Irelia with 4 out of 6 tanking items still does a large amount of damage.

Ghost for easy kiting, escaping and chasing.
Ignite for a counter against healers/regenerators like Soraka, Sona, Sion, Warwick, Mundo and Irelia.

I define two groups of summoner spells, ego spells and team spells.

Ego spells: Flash, Ghost, Revive, Teleport and Cleanse.
Team spells: Exhaust, Heal, Smite, Fortify, Clarity, Clairvoyance, Rally and Ignite.

Ego spells are spells that mainly benefit yourself and therefore don't have a large impact on the game. Team spells benefits both you and your team and tend to have a huge impact on the game if used right. Idealistic as I am, I want to pick what my team needs. I suggest you do the same. ;D

I add this AP section to this guide to show you another fun way of playing Irelia.
Irelia is classified as an assassin, I think that means she can burst down an enemy really fast.
The way to optimize her burst damage is by using an AP build on her. Again, Sheen is her core damage item. Upgrading Sheen to Lich bane will have her do mindblowing burst damage.

EDIT: Since patch v1.0.0.113 Equilibrium Strike's AP ratio has been decreased to 0.5 from 1.0. This hurts, but it's not dramatic. The Lich Bane + her ulti still work wonders.

Marks:
Greater Mark of Desolation. You might be building an AP build, but auto attacks enhanced by Lich Bane still do physical damage.

Seals:
Greater Seal of Scaling Mana Regeneration. Your normal auto attacks won't do a lot of damage, so you never want to run out of mana. The regeneration these runes offer you is pretty much all you need. AP Irelia isn't as mana-hungry as most AP-based characters.

Glyphs:
Greater Glyph of Scaling Cooldown Reduction. Using skills more often results in more Lich Bane procs.

Quints:
Greater Quintessence of Movement Speed These runes combined with the Quickness mastery and lvl 1 boots results in having 409 movement speed at lvl 1. You will outrun everyone, probably even Rammus. Never underestimate the power of speed. If you're planning on playing passively (which you shouldn't), get health quints.

I take both Archaic Knowledge and Sunder in the offensive tree. An Irelia using a Lich Bane will end up doing equally much magic damage as physical damage (again, auto attacks enhanced by Lich Bane do physical damage).
In the utility tree, the most important masteries I pick are Haste , Awareness and Quickness . I explained before why these are important. You'll probably need that extra 3 mana/5 as well. Utility Mastery is just a great bonus, not a necessity. I also pick Mystical Vision , which I will explain later.

The first items I get are Boots. 409 movement speed? Yes please. Another reason for getting boots is to be able to buy 3 potions for an even easier laning phase.
The next item is, of course, Sheen, no more explanation needed.

Back when Equilibrium Strike had an AP ratio of 1, it would be better to buy a Needlessly Large Rod before getting the Mejai's Soulstealer, because it's more reliable. However, now the AP ratio has been reduced to 0.5, the effect from the NLR is hardly noticable, so I suggest getting Mejai's ASAP to utilize the kills you will be getting even more.

If you have over 1600 gold at this point, see if you have more than 5 stacks on your Mejai's. If you do, go ahead and save up for (or buy) your Lich Bane. If you don't, buy the Needlessly Large Rod first. The reason you should do this is the following: it's useless to upgrade Sheen to Lich Bane if you don't have AP. When you don't get AP before buying a Lich Bane, chances are you're doing the same amount of damage as you would with a Sheen.

Now you have a nice amount of AP, we're going to utilize that even more by getting a Lich Bane. By buying this item, your burst damage will skyrocket. And because Irelia's ultimate can proc Lich Bane 5 times, you also have a steady and huge amount of DPS.
Next, we upgrade that Needlessly Large rod you got earlier into a Rabadon's Deathcap. The reason I don't want to do this earlier is because of it's price. It would delay your Lich Bane purchase.

If your enemies managed to survive long enough to make it to late game, you are now about to finish the game. After you got your Nashor's Tooth, even the meanest tank will be terrified of you. You should buy Nashor's Tooth for the following reasons. It will actually make your W skill useful and it gives you 25% more Lich Bane procs. You also need the mana regeneration, now that you almost hit the cooldown reduction cap.

The last item I used to get was a Void Staff, but since patch v1.0.0.113 was released, AP Irelia's skills have become significantly weaker. Therefore, Void Staff's effectiveness has been reduced as well. Getting a Hextech Gunblade would make more sense than a Void Staff. The Gunblade is a great item because it adds one heck of a lot of life steal. Together with your ultimate and the Nashor's Tooth, you're going to rape the raped faces you probably already raped before, but harder.

I use this item build to maximize Irelia's damage output. I don't get any survivability items because you really shouldn't need them. With Flash, Irelia's speed, passive and lifesteal (ultimate) you should be able to get out of just about anything. If you fail at this, sell your Mejai's and get a Banshee's Veil.

You want to nuke your enemies to oblivion, so first of all, you should max Equilibrium Strike, it does 300 base damage at level 5. Once you have a Lich Bane, your Bladesurge's power will improve significantly, so maxing that after your E would make sense. Also, lvl 5 Q has a very low cooldown, so it makes you extremely mobile during teamfights. I max Hiten Style last because I'm not getting attack speed items until late game and lvl 1 W and your ultimate provide more than enough health regeneration. You should put a point in Transcendent Blades whenever you can, because its base damage increases a lot with each level.

AP Irelia is very squishy, so I pick two summoner spells to help me with that. I like using Flash to get out of a "sandwich" situation and I like Ghost to counter-act slows and improve kiting.
As you could have seen before, I choose Mystical Vision from my utility tree. Clairvoyance is the best spell to prevent getting ganked. It has a very low cooldown so you can spam it every time you want. It is also very valuable because nobody in your team will need to facecheck bushes, 'Knowledge is power'. You can also use Clairvoyance when ganking. Make sure you always have an eye on your prey, do not let them use the bushes to decieve you. You only need one Clairvoyance per team.
Another useful spell to escape from a gank is Cleanse, but I would only pick this in ranked games and I see the enemy team has an overkill of CC.

Make use of your Sheen! Alltogether, Irelia has 8 ways to proc it. (Q, W, E and 5 times R). Wait until Sheen/Trinity Force/Lich Bane comes off cooldown before using a skill.
When destroying towers, use your ulti, E and W to proc Sheen to make towers melt, this works incredibly well with AP Irelia.
You can use a low health creep to cross a large distance in a fraction of a second by using your Q skill, use this to gank or to escape a gank.
When you're in a teamfight, stop the enemy from killing your squishies by using E.
When you're in a teamfight, kill the enemy squishies first.
When you're in a teamfight, prioritize killing squishy carries over killing squishy supporters.
When someone in the enemy team is legendary, kill him.
When someone in your team is legendary, protect him with your life.
Keep enemies from farming.
Farm whenever you can.
Don't get ganked.
Gank.

Please help me make this guide even bigger! :D

Updates (dd/mm/yy):
05/03/11: Added AP Irelia build, updated Other Hints&Tips and Summoner Spells for AP Irelia (how to use CV).
16/03/11: patch v1.0.0.113. Equilibrium Strike got screwed. AP Irelia is still viable.
18/03/11: Switched the item order on AP Irelia. Mejai's can now be bought earlier because the AP ratio on Equilibrium Strike is now cut in half anyway. Replaced the Void Staff by the Hextech Gunblade.
06/04/11: Switched back to comment-to-vote. I noticed too much downvotes and none of them actually felt the need to say why. This should fix things.
23/04/11: Switched from Long Sword to Doran's Shield as a starting item. Updated several general things because of recent patches.