there are a few irc clients that run in the terminal, but for most users (like me), gui is the way to go. ircle has about every feature that you could ever want and several years of mac-only development behind it (i think it's about $15 shareware, but you get a 30 day trial perios). upk is a new mac os x only irc client written in cocoa and providing a nice feature set without the desktop clutter. jediknight and athenalirc are also available, as well as akwairc...
why am i writing this? just search for "irc" on version tracker and test a few out. upk and ircle are my favorites (upk is cool and free).