Growl integration is optional, but can’t be a resource hog
Never used one, but I found this…
Colloquy is great. (And free).
I don’t do a ton of IRC chatting on my Mac, but Colloquy is what I’ve used too. Never had an issue with it, and the price is certainly right. :-)
+1 for Colloquy also chatzilla is a firefox add on, if you would rather go that route, not as pretty or solid feeling as Colloquy though.
mIRC won’t work?
The Adium beta has IRC support which I’ve used and find decent. I must admit I’m no IRC power user though…
Another vote for Colloquy. I’ve used it quite a bit in the past and really came to like it.