Both Pidgin and Adium are based on libpurple[1], which does support IRC.

1. http://developer.pidgin.im/wiki/WhatIsLibpurple

"Pidgin is compatible with the following chat networks out of the box: AIM, ICQ, Google Talk, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, Novell GroupWise Messenger, QQ, Lotus Sametime, SILC, SIMPLE, MySpaceIM, and Zephyr."[2]

2. http://socghop.appspot.com/org/home/google/gsoc2009/pidgin