Irelia is one of my favorite champions to play. She is very fun and good for dealing lots of damage in a short period of time. With the build provided you are given durability and lots of burst damage. As Irelia you want to seek out the squishier targets and take them down first.

With Irelia you rely a lot on your build. You kind of want to be a little bit lower in your hp percentage so you can stun with your Equilibrium Strike. Due to this you will be able to go in and get farms with your Q ability and if you take a little damage that will be okay. The doran's shield at first is to prevent you from dying while attempting to farm. Like I said you want to use your Q to farm as it will get you in fast and you will be most likely to escape without your enemy getting a chance to react.

Irelia is very good at low hp. You HAVE to keep in mind your damage with your abilities as well your mana when fighting at low health. Your passive on Hiten style will give you ability to fight and survive. Another thing is the stun that comes with Equilibrium Strike. The comination of the two makes irelia a great lure and gives her the ability to survive in 1vs1 fights. Make sure your target is lower HP percentage before casting your Equilibrium Strike or you will have a tougher fight on your hands. Along with those abilities you have your Bladesurge. Bladesurge will allow you to jump to an enemy minion if you end up getting cuaght overextending or chasing. Also your ultimate, Trandscendent Blades is great for finishing off an enemy champion if they try to run.

I like to max out Irelia's E first for a lot of damage and the stun durration. Her E I mainly use for the HP it give on basic attacks, but late game the true damage is great. Her Q is great for damage and the jump, but early game I mainly use it to close gaps between targets to use Equilibrium Strike.

I like to close the gap with Q and then E and W with auto attacks until I get hurt or they run to turret or ally champion. With this early game it is tons of damage. Late game i close the gap with Q and then activate W and auto attack, if i get low i use E to stun and finish the kill.

I like to have early game survivability as well as late game damage. I start with Doran's Shield to ensure I can farm without being destroyed. Mercury Treads are great in case of ganks and for ganking mid. Youmuu's Ghostblade is great all around. It gives you cooldown reduction, early critical damage, and attack speed. Phantom Dancer will grant even more damage and movement speed. Banshee's Veil is great so you can target squishy dps champions and slightly ignore AP champs. Hextech Gunblade gives survivability and boosts damage on E+ULT along with attack damage. Infinity Edge is great for getting in and Annihilating Health. You will destroy people without them knowing it ever happened.

In the mastery tree I like to take the 9/0/21 because of the cooldown reduction and extra experience/gold. As for runes I like to have the early game crit chance for extra damage and magic resist on assasins is always good. the cooldown reduction will help with bursts of damage. The armor penetration per level is great for late game when those pesky tanks try to ruin your penta kills.

I like to take exhaust for early game kills. Flash is to survive ganks, fed champions, and just people who are killing you. Flash is also usefull to catch people too. If you are out of range for your Q you will easily get into it with a quick flash. Exhaust is great for escaping too, or helping your team mate escape.

Pros:
With the build you will be able to kill quick
Irelia has tons of mobility
You can survive at low hp
You get great early game survivability
You can survive and destroy Health
Cons:
Late game you are squishie than most champions
Early game you Don't hit very hard with basic attacks

As Irelia, in team fights your job is to get in, kill the huge threat, move on to next threat or get out before you die. You want to focus the carries and squishy champions. If you go for Tanks or Fighters your hp will drop.