its more the other way around, lifesteal is very cheap and usualy in great quantity, thornmail may slow it down but wont counter it, lifesteal however will counter thornmail.

You also have to account for the Champion you're playing against on whether or not this will work... for example, when I played Trynd, I'd get two Blood Thirsters and stack them up asap.

This gives me 50% life steal and 200 damage, not counting other items and the like.

If we add in the other AD items I used:

Infinity Edge - 80
Cleaver - 55

Add in the factor of his base damage at 18 being 125 (128 with mastery)

Then 25 from Bloodlust. We have... 488 AD.

Adding in a Phantom Dancer and Infinity Edge... well, let's just say it's likely he'll be critting a lot.

So before armor he'll deal 1220 damage on a crit with all of this.

He'll heal himself for 610 HP with each crit, it'd basically take him four crits to heal himself to full.

And this is only a 1v1 scenario, if it's a team setting there's always the possibility another AD champ picked up Starks which would bring it up to... 70% life steal. It'd take 3 crits before armor to heal himself to full.

If the only armor you have as Irelia comes from base, Thornmail, Atmas and from what I've seen GA that's:

295 armor at 18.

This gives you around... maybe 60%+ damage reduction.

With runes, the mastery and BC, then Tryndamere will bypass 76 of your armor taking it from 295 to 219.

So you're still in the 60% range.

Which means his crits would only hit from about 732.

Going with just the thirsters he'll be healing for 366 each crit, if we add starks it'll be 512.

So, with just the thirsters, he effectively negates your Thornmail as 30% of the damage off his crits are going to be 366. In a team setting he'll be gaining more health than he loses from whacking you.