Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Irelia is a solid counter-carry. Her primary role is to shut down and threaten squishies in the back with her insane mobility and burst. She is much less gimmicky then Akali or Poppy, two other champions that share the same role as Irelia (counter-carry, mobility, burst). Meaning if you enjoy playing Akali and Poppy, you will see much greater success playing Irelia.
There are no better alternative to the Armor Pen marks.
Consider HP/Level seals, but due to the HoG changes, the flat Armor is really sexy early.
MR/Level glyphs are the strongest glyphs.
Consider Flat HP, Movespeed and Flat ARP Quints. The 8.1 Hp/5 from quints is very powerful, however if you don't need the Regen or maybe not even have them, 10 ARP and 4.5 Movespeed are also very strong.
0/9/21 is the only Mastery build I would ever consider. Presence of the Master is a necessity. Strength of Spirit gives you the little bits of HP regen you need while laning. Meditation is also necessary to keep your Mana up while using Q to creep. 6% CD, 3% Movespeed, 6armor/mresist are all excellent.
Doran's gives you everything you need to lane vs a AD harasser, and is strong for level 1 fights.
Regrowth Pendant gives you insane HP/5 vs AP harassers, and lets you rush Philosopher's Stone.
Boots 3xpots allows you to chase and be aggressive.
Sapphire Crystal 2xpots lets you rush the Sheen to have early burst damage with Bladesurge.
Ignite is very important. It synergizes with her burst and makes laning against her very scary.
Ghost is better than Flash because Irelia can sustain herself in the fight to take full advantage of Ghost duration. Ghost also makes Irelia practically invincible, able to escape from 2~3 man ganks with her passive.
Farming is reeeeaaally important with Irelia, your early game goal is to stay in lane and farm farm and then farm some more.
Early game goals:
Mid game goals:
FoN vs AP heavy, Omen vs AD heavy.
Force of nature - Randuins Omen
Whichever you didn't finish.
Atmas Impaler or Guardian Angel
Atma's for more damage, or GA for more survivability.
Wit's end is very strong if the enemy team is tank heavy. The attack speed and proc synergizes very well with Hiten Style.
If you're against insanely fed AD's and you can't burst them down anymore, Thornmail is always an option.