who do you think would win?

whoever has biggest titz

He will deal good damage to her but she is tanky enough to take it and sustain back up, she also punishes him heavily when he harasses. pre-6 it's a bit in rengar's favor but after that I'd put my money on irelia in a straight fight, he's pretty squishy after this 3 second boost falls off and cc really screws with his rage generation.

If nothing else she'll be lower guaranteeing the stun when the junglers comes as he has to heavily push his lane to keep his sustain up.

Pre-2 irelia can engage him whereas he can't afford it.

Sometime around level 3 irelia can still engage him but they can more or less mirror each others survival strategy except irelia can perform more easily and can farm much faster. Ren has to keep the pressure on irelia or his team could lose time. Losing time against irelia is no different from feeding. At level 5 rengar always seems to have some good chance of being able to kill irelia, but once irelia reaches level 6 she has enough utility to just counterplay rengar all day long. Its not even a question of who's stronger after that. If its 1v1 rengar will likely just be fighting her all day long with some build that keeps him in play. But so would irelia only hers will be better suited to her. I don't know if rengar would fall off eventually for this reason but when you do weird stuff with your build to counter a specific champ you're either up to the task like irelia or you fall off.

I have no idea what you just said it really didn't make sense.

He's basically saying that rengar vs irelia is pretty much a stalemate as long as you can survive his level 2 burst.

Irelia can beat him but rengar has to be making a lot of mistakes.

I have lost to rengar who was lv 2 because he used Empowered Savagery to start the fight with me resulting in me being down to about half health after a single hit.

Empowered Savagery is a lot like a triforce proc but it is all damage instead of just base damage.