I know nothing about Macs or Mac software having never used a Mac, though I have been told that ircle is the best Mac client currently available. I have found some Mac IRC client download sites:
Macintosh IRC: http://www.uvm.edu/~tday/MacintoshIRC.htm IRC for Macintosh: http://www.ex.ac.uk/~jastaple/irc/irchelp.html