Internet Relay Chat
Adium does not currently support IRC.
We do have a plug-in that adds support, but it is incomplete and not ready for general distribution. IRC support will require much more work on Adium's general support for multi-user chat before our attention will turn specifically to IRC. In particular, we don't have user interface for most of the IRC-specific features, such as modes and topics.
But Pidgin supports IRC!
Remember, Adium is not simply the Mac version of Pidgin. We use the same core library, but the rest of Adium and the rest of Pidgin are totally separate programs.
The IRC plug-in mentioned above uses Libpurple's implementation of the IRC protocol, but the necessary user-interface improvements must happen entirely within Adium, and that is not a priority at this time.