Wow! Did this got popular over night? O.o

I normally start with tear, then boots 1, sheen for the proc, and then evaluate situation.
Balanced fights??? Finish triforce and manamune
Heavy casters?? Build BV, then manamune
Heavy phisical?? FH, and then finish manamune.
Owning ???? I have found a youmuus to work wonders with CDR AS and AD.

Start with D-blade and an hpot. Ignite/ghost as your summoner skills. Level EQWER, R>E>W>Q. If possible, grab a solo laned against the enemy ranged carry. Irelia is brutal against ranged carries in the early game. Once you get to level Q, dash at the enemy carry and start beating on them. If your health happens to drop below theirs %-wise or if they run, then use E for the stun/slow. Rinse and repeat. If the enemy is close to dying, use ignite to seal the kill. It's really pretty easy.

Item progression afterwards is boots => phage => sheen => zeal => triforce => merc treads. Irelia is a really good farmer. If you're farming well and are able to get a hero kill, you should be able to finish it within 20 minutes. At this point you will be able to do a lot of damage and should be stronger than most other players. Abuse your advantage and gank wherever possible. Don't just sit and farm creeps. Go farm heroes and towers! Irelia is at her strongest during the midgame from ~ levels 6-12 or so.

What you get after triforce and merc treads depends upon how the game is going. If you're doing well, you can probably afford to get another damage item like a black cleaver or starks. If not, you may need to invest in defense at this point, like a banshee veil or randuins. If you're unsure, get starks. The added attack speed and leach help boost Irelia's survivability. Keep in mind that attack speed is more important than attack damage on Irelia because of her W.

Ive seen AS work the best with Madreds and W active

i usually just build her a tanky dps... with Warmogs, Frozen Mallet, Atmas then 2 last defensive items. Works out REALLY well for me, i get in their face and be a real pain in the ass, more hp makes you even more deceptive with her e, but wat i really miss is probably lil atkspeed... i would suggest getting a wits end as her mgc res defensive item... When irel has her w on, her AD + True DMGE+ Mgc dmge form wits end i trotally abandoned the Tri-F didnt think it was worth it....

Building Irelia as a tanky dps is the way to go... anyone who says otherwise is playing a champ that is probably not very useful to their team past the 30 minute mark, whereas if you build tanky dps you can carry your team to win and absolutely DESTROY the enemy carry.

Good Build (Cookie Cutter):
0-21-9 Masteries
ArPen Marks + Quints, Armor Seals, MR Glyphs
Spells: Ghost + Whatever

Item Order:
1v1 Top: Crystal + Pots, 1v2 or 2v2: Dorans shield
Lvl 1 boots
Sheen -> Triforce
Merc treads (Only thing better than CC reduction is more CC reduction)
Build atmas and BV at your disgression... always take a health crystal first though even if you're building atmas.

From there evaluate the situation and start building things like FoN for MR (the movespeed is oh so sexy) or Randuins for Armor.

The reason you DONT need to build irelia as a glass cannon should be obvious, bladesurge. If you're smart, you'll be bladesurging to the enemy carry and smacking them with EQ strike right as the fight starts, and taking them down easily afterwards, who needs tons of AD/AS to kill a carry? Also, this build is entirely tanky, but also has high mobility (triforce) and some AD (triforce + proc, atmas).

One good tip to use with irelia is be sure to combine your ult with triforce mid fight, if you get a swing in every time you cast a blade your damage output goes way up.

Nice necro.

umm mind if i throw my two cents in?

runes i use armor pen red, flat armor yellows mr per level blues and movement quins.

I seen someone say get doran's blade first this person is right on the money.
then go boots, push for sheen, phage, then i get hextech revolver. after i get that i push to finish trinity force. after that is done i go for the pick axe.

edit: oh yes i go 0/9/21 for masteries works well btw