Zircon is also an IRC client, written by Lindsay Marshall.
It was the first GUI IRC client I've ever used on Linux. (I have used some graphical IRC clients in Windows, but those sucked. And no, I haven't used mIRC at all =)
The toolkit it uses is Tk, and it was written in Tcl. According to the hype, it supports all ircII features and more.
I don't remember much about it, but I remember one curious feature: Plain <ENTER> sent a line, while <Shift+ENTER> sent the line as an action (this is done in other clients with /me).
Home page as of writing: http://catless.ncl.ac.uk/Programs/Zircon/