To counter tankiness you have to make the most out of your Hiten Style's 6 seconds. Which means you build a lot of speed items. Wit's end is a very good example, and it also adds someof its own damage per attack. A better version of wits end is Madred's Bloodrazor, it adds %4 of the targets health as magic damage to your attacks. But to be honest you will rarely need these items, irelia's job is to kill the carries, if you can get to them and finish them off then you have nothing else to worry about.
Do note that since madreds and wits end do magic damage, they will be useless against a morgana, you'll barely be able to break her Black shield with your attacks.