Heyah folks! :D I've been playing Irelia basically since I started LoL, and I've tried a bunch of different builds. After a few revamps, Irelia is now IMO best suited to be a solotop offtank.
Feel free to make suggestions, all constructive criticism appreciated. Let's get started, then :D

My runeset haven't really changed much since I started playing her, and looking at some other high-rated builds, these runes seem pretty much staple. Armor pen for more consistant damage, armor and MR for sustain in lane. The quints, however, I have experimented with. What I figured out was that having that 5 extra AD at the start really helped with the lasthitting. I mean basically, it makes it easier to get the reset on Bladesurge AND lasthit with basic atts.
The 1 mms quint is convenient because with 320 base you can outspeed anyone that doesn't also have mms runes, which means you'll be able to harass better with basic attacks.

Since we're aiming for an offtank role here, I ruled out the offense.
Both utility and defensive masteries give so much sustain that offense just isn't worth it in sololane.

Again a complete revamp of the build I did earlier, but Sheen is still VERY important.
It's also good to have the mana from SC at level 1, as a hard matchup in lane means you mainly lasthit with Q. This gives you barely enough for one of each pot, too.
I prefer going directly for Sheen the first recall, but if you can sustain for long enough to get the 1210 gold, get boots too. (Or if you can't afford Sheen, just get boots, nobody wants the tome anyway).
Boots are very hard to choose for Irelia. Mostly everything goes well with her. Ninja Tabi is commonly used for AD offtanks, but ATM I prefer the greaves because they're cheap and let you W creeps faster. Swiftness and Lucidity are also viable choices. Avarice blade is next, because an early G/5 item is awesome.
As for the rest of the build, you will have to consider the circumstances. Many people, mysel included, like getting Wriggle's in solotop. Obviously it's convenient, I mean:

- A little bit of AD
- Some lifesteal (to stack with your W aswell)
- Armor, important for an offtank
- Lets you farm SO DAMN FAST!
- Gives you a constant ward for the bush in the river

That's definitely worth the money, eh?
So then obviously, you upgrade Sheen to Trinity (could do this earlier if you have the gold), and the Avarice blade to Youmuu's. The last item is COMPLETELY situational. I put the IE in there because I already have Zerk, and to bump up your damage as much as possible in one item.
You could also consider stuff like BVeil, GA or Randuin's.

This has been discussed a lot lately. Some people like to go E-Q-W, others W-E-Q, and people like me who like to gank their carry a lot (>:D) go with Q-E-W.
Basically my line of thought is that since Q procs Sheen, the gain of 20 more damage per point in E isn't really worth it. What I usually end up doing is hitting them once with Q and then go back, unless it's a gank. Also, E is really useless for farming, which is... not so good in solotop. xD

Also debatable, I used to play Ignite rather than TP, but lane consistency > all. lol.
Anyway I really want people to get that Flash is NOT a good spell for Irelia to be using. With the amount of sustain from W and R as well as the fact that she can distance-close to minions after attacking someone, it just isn't useful. Ghost is way better for an offtank with a closer.
I have also seen a lot of exhaust, which is fine because she's an assassin without a stealth, but I prefer covering the spots she doesn't already have. (slow from E)

A very effective build for farming top and outlaning whoever they send. Getting tons of lasthits before initiating teamfights is ideal for any toplaner. Mind that you should always return to your lane to farm as quickly as possible after ganking, etc. And remember that even if you build Irelia as an offtank, she's still an assassin, so their carry shouldn't be too hard to take down with abit of overextension.

That's all folks! :D