Wallops is a light-weight IRC client with an emphasis on compatibility. It runs on Macs where ShadowIRC or Ircle won't, namely oldies like a Mac Plus. It does, however, also run fine on Mac OS 9 machines. Version 2.0 now comes with a lot of changes, including (finally!) resizable windows, tabs and additional commands. Find the full changelog and download at the author's website.

Edit. Version 2.1 has since been released, containing a couple of bugfixes and enhancements.