Based on the *nix IRC client
irssi, MacIrssi is an powerful, free of charge, open source IRC client. It combines the power of the irssi engine with an elegant and flexible GUI. See also
http://www.irssi.org/?page=about for information on the irssi engine. Latest builds, once again, support MacOS X 10.4 and 10.5 on both Intel and PPC platforms.