I have seen a few threads about IRC on macs and know you have to pay for them normally.

I found http://xchataqua.sourceforge.net/

X-Chat Aqua is a MacOS X IRC client. If you are familiar with XChat for UNIX/GTK+/X Windows, then you will be at home with X-Chat Aqua. X-Chat Aqua uses the IRC engine from XChat and has been designed to look and feel like XChat. As time goes by, I expect that I will add features that are not necessarily part of XChat.
Dunno if it's any good for any of you. Thought I would post it just in case.