This guide focuses on Irelia being an off-tank assassin type champion. She can also be built as an AD Carry, but this is typically not as viable. A tanky Irelia wins games, and she typically wins them fast. I encourage you to not follow this guide to the letter, as Irelia is a very fun champion to experiment with, especially after you've gotten good with her. That said, the items I suggest above give what I consider an excellent balance of both types of damage mitigation, while letting you still pump out pretty beefy damage.

Pros:
Strong damage early, mid, and late game
Not particularly item dependent
Awesome survivability, even with defensive items
Very few champions counter her well ( Jax, Jarvan IV, Singed, Pantheon would be her weak points IMO)
Fast, good escape ability with Ionian Fervor/ Mercury's Treads/ Juggernaut , essentially can't be CC'd
Extremely good farmer/last hitter
Closes distance ridiculously fast
Great laning ability, the ideal solo top character
Good ganker
Excellent kiter, good comeback character in 1v1 fights, also makes her an outstanding baiter

Cons:
Can be mana intensive early game
Pretty much needs solo lane or heavy feeding to dominate

For Irelia, I love armor penetration runes for marks and quintessences. You get a massive increase in your damage right at the beginning and these runes will give you a solid chance to gank early.

For glyphs and seals I would suggest flat armor and magic resist to guard against any harassment you could have to deal with early on. Other possible choices include CD Reduction, HP/level, dodge, or flat HP.

Juggernaut , along with Mercury's Treads and Ionian Fervor, give her potentially an 85% reduction to almost all forms of crowd control. The tree also goes well with the defensive runes I suggest (flat armor/mag resist), and it even gives you some cooldown reduction and extra gold. Grab Swiftness from utility to make Irelia cruelly fast, she has one of the highest base movement speeds in the game, take advantage of this fact! Then, Improved Summoner Skills in offense(5 AD/AP for ignite being on cooldown is solid for 1 point).

For summoner spells, I prefer to go with Ignite and Teleport. On Irelia I consider Ignite to be essential. As soon as you get Sheen, a strong combo followed with Ignite should finish anyone even remotely squishy. I choose Teleport because it allows you to be somewhat sloppy at the start and teleport back to resume farming. Teleport is also solid late game for jumping into team fights, defending turrets, pushing, etc. Flash, Ghost, and Exhaust are all good as well depending on your play style and preferences.

Spend your points based on how you are performing in your lane. If you are getting harassed, go mainly in this order: Transcendent Blades> Hiten Style> Bladesurge> Equilibrium Strike. If you are dominating your lane: Transcendent Blades> Bladesurge> Hiten Style/ Equilibrium Strike. Pumping points into Hiten Style early on can be very powerful against harassment and keep you laning indefinitely even against the most difficult opponents.

I'm very flexible in my item choices on Irelia and these items are in no way final. Irelia can be built many ways and experimenting on her can be a lot of fun. The only two items that should be in EVERY Irelia build are Mercury's Treads and Trinity Force. I prefer to start the game with Boots and 3 Healing Potion, followed by rushing Sheen. Vampiric Scepter is a good early game purchase as well as it gives you immense staying power in your lane, which is vital in soloing top. Build Mercury's Treads first unless you happen to have enough gold to just buy Trinity Force early.

At this point you want to analyze your opponents. Just take a quick peek at their items and team composition. If they're AD heavy, I like to get a Chain Vest then build a Warmog's Armor and Randuin's Omen. If they're AP heavy, Negatron Cloak, followed by Force of Nature and Warmog's Armor. Guardian Angel is always a good idea, and I usually build it after Warmog's Armor. If you plan to turn your Vampiric Scepter into a Zeke's Harbinger, then turning it into Emblem of Valor early would be wise as it's very cheap in midgame when gold should be easy to come by.

For a final item there are a few good choices. I like Zeke's Harbinger, as you are pretty tanky at this point and should be initiating team fights/fighting with your team, so the aura can really help your team push (Also, Irelia has a very slow basic attack speed, and the increase really boosts the lifesteal and damage of Hiten Style and auto attacks, as well as other lifesteal effects). If your team is pretty low in the AD department and the aura might be a bit useless, other good choices include Hextech Gunblade and Bloodthirster. You could also get another defensive item, but getting any of these items can practically turn you into a carry.

The build I suggest at the top is best against a balanced team of AD/AP champions. The final build I go for in most games is Mercury's Treads, Trinity Force, Warmog's Armor, Guardian Angel, Banshee's Revenge, and Zeke's Harbinger. With this combination of items you reach an amount of armor and magic resistance that it really doesn't pay to go past. Anything past the 60-70% reduction range and the reduction per point of armor or magic resist tapers off badly. You also have around 3500 HP and are generally a massive pain in the *** to kill.

Other items I've used in the past that I consider good for Irelia include: Shurelya's Battlesong, Spirit Visage, Phantom Dancer, Black Cleaver, Nashor's Tooth (Great early on but you will have mana issues), and Atma's Impaler. Play around, find a combo that works for you.

First and foremost, DO NOT waste all your mana Bladesurgeing minions. Keep at least 400 or so mid to late game for team fights, and try to keep it around half early on. The difference between a good Irelia and a great one can be how well they manage their mana.

You should ideally be soloing top with a jungler (I prefer Gangplank or Shaco), coming by to help you gank. Another reason I like Teleport as it allows my jungler to keep farming and not worry about defending your turrets. You will want to turret hump as much as possible early. Mid to late game, you should be obtaining some solid defensive items that will let you initiate team fights and walk away. If you push the enemy back to their turret (yes you will do this quite often in 1v2), it can be a good opportunity to run down to mid and gank.

In 1v1 encounters, you will want to drop the following combo: Ult, W, jump in with Q, E. Then you run away and kite them while blasting them with your ult and waiting for your cooldowns to come back up. When Bladesurge is up, jump back at them and finish them off, or rinse and repeat. Typically you probably will be able to stand toe to toe with most enemies and kiting them won't even be necessary. More often than not, they will probably run away from you as well as a lot of players/champions struggle playing against Irelia.

Bladesurge can be used to close massive amounts of ground in seconds. I suggest going to keybinds and finding smart cast ability binds, and changing at least your shift Q to Q. You will then cast it on your cursor location, reducing the time it takes to target enemies and click twice. If an enemy is far away, jump to a low health minion and jump again to catch them offguard and gank them. Similarly, when running away, jump to minions to separate yourself from your pursuers. With proper use of Bladesurge, Irelia has arguably the best mobility of any champion.

Hiten Style (both the heal and the damage portion) work against turrets, unlike most lifestealing effects. Take advantage of this when you can, you can tank a turret for a long time late game.

Don't go oom farming creeps with Bladesurge. I know. It's fun. Still, don't. Take it easy and grab some kills with it, try to find medium health minions that you can't last hit with a melee, but can with Bladesurge and use it on them. Otherwise last hit with auto attack and conserve mana for fighting players. Never use your W on minions, it's just a waste. Transcendent Blades can be used to push and get a huge wave of creeps down, but likewise, it's typically a waste to do so.

Farming some jungle mobs while waiting for your allies to respawn or gather can be efficient as well, but Irelia is a mediocre jungler and you won't want to focus on doing this. Keep auto attacking and regenerate your health as much as possible when in lane.