Noise?

The noise made me pretty uncomfortable.

I felt a bit uncomfortable but I watched it on DVD. Maybe in a cinema it would be more intense.

https://www.youtube.com/watch?v=ZuokWUhMGQc

And no, watching it on DVD won't give you the intended experience, unless you have a crazy system. It hovers around 15-20hz which is just barely audible and it's not actually playing against the soundtrack. It's just bursts here and there so that again softens it. If it dipped down to say, 7hz then yeah. People would have been vomiting all the way out the door.

The way it's used in Irreversible, the effect is to just make you feel a little uneasy by tightening your intestines a little. It doesn't mess with your nervous system so it doesn't raise anxiety as far as I know. The constant flashing of red colors is what arouses anxiety and panic attacks, and the tight hallway (they're actually just walking around in circles) is intended to make you feel claustrophobic. Not to mention you, the viewer, are confused and suddenly thrust into this harrowing world where you have no idea who is who and what is going on. All of these factors, plus the sound, are why you feel sick during the opening. It's not just the sound itself.

Great film making. The atmosphere in this scene is unchallenged.

Also, "Earthquake" is another film that utilized this technique. Although like Irreversible, I don't believe it works without a great system or in a theater.