ShadowIRC is a fast IRC client for the Mac OS. The program is extensible with C-coded plug-ins compiled with CodeWarrior. This makes ShadowIRC plug-ins faster and more flexible than AppleScript-based scripts, though you will need knowledge of C and a copy of CodeWarrior if you are going to write plug-ins. Among other features, ShadowIRC can process Ircle and mIRC color codes (though not at the same time). The source code is also available.
What's new in ShadowIRC
Version 1.1 fixes the following:
Fixed a potential crash when dealing with out-of-range color codes. This could manifest itself as a crash on entering a channel with certain color codes set as the topic.
Other... item in the font menu to allow choosing font sizes not listed.
An excellent irc client. It doesn't have all the flashy features you get with Snak or Ircle but it's much smaller and a lot faster. Also, I find it handles colour much better (useful in 'quiz' channels).