"Based on the _libpurple_ protocol library, Adium can connect you to any number of messaging accounts on any combination of supported messaging services (see further down for the list) and then chat with other people using those services." (Emphasis mine)
"Libpurple is the library which provides network-level connectivity for most services in Adium. Like Adium itself, it is licensed under the GPL. In addition to Adium, libpurple is the core of the Linux and Windows IM clients Pidgin and Finch. (...)
Libpurple was previously named Libgaim. "
So yeah, Pidgin is an option, because neither Adium nor Pidgin speak IRC, libpurple does. The support should be ~the same~, of course depending on the version of the library used.
I wanted to switch to Pidgin a while back because it had support for the kind of encryption I wanted (public key) and Adium didn't, but I ended up not doing it and I thought the reason was because Pidgin doesn't do IRC.
"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]
Yep: Pidgen has supported IRC so long that back when I was still using it, and the project hadn't yet been renamed from Gaim, it already supported it; it had a few stupidities, and didn't support userhost-masks for buddy bindings (something that simply "makes sense" if you use IRC enough), but it definitely worked and still does (I have friends who use it all the time for IRC, and one who even contributes patches to it).