XChat Aqua/Azure is OS X native front-end of xchat2 written on Cocoa rather than GTK2.
Many people like the retro, minimalist look that harkens back to the old days (I’m talking 1990s) where multiple channels are streamed together into a single window, and people relied on typed or shortcut commands to do most things.
This is a radical change from just a few years ago, when the best apps were shareware worth $30 or more.
All of these apps are stable and usable, and that reflects not only the multi-year track record of these particular programs, but the overall maturity of IRC itself.
Open source freeware, available for Mac OS X and i OS. They’ve been around since 2004 or so, and have grown from a lightweight client into a stable, full-featured one that supports not just IRC but also SILC and ICB.
This is more than most users will need, as you can transfer files, chat on multiple networks, plaster your screen with m IRC colors, and handle scripting via Applescript.