X-Chat Aqua is a faithful reproducition of Xchat for X11. This means its UI is rather clunky, but the functionality is top-notch. the scripting interfaces allow endless customization, and the client itself is stable and fast. It has a few OS X specific extras, like dragging files into the nick list to do a dcc send and an optional 'metal' Interface.
Version 0.14.0 adds working support for nickservs and a UTF-8 mode that has proved to be fully compatible. In short, this is the best mac IRC client, though if you are a casual user, the extended features may not be useful. However, it is free, while Snak and Ircle are not (albeit, they are cheap and I own both, but I USE X-Chat Aqua).
NB: Older versions of X-Chat Aqua for OS X 10.2 no longer appear to be available, but in fact are; they are just hidden rather well. This link should take you to the full list of past releases.