Reply-to: "Perl Mongers of Omaha, Nebraska USA" <omaha-pm@pm.org>
If you want something graphical, I have used Colloquy. It's pretty good. I would recommend against the OS X port of Irssi to the GUI. It seems to be very buggy and not very well maintained. Normally, I also use irssi, but Colloquy is handy in that it integrates with OS X notifications and such. It's a little less intimidating than irssi can be if you're not used to programming your way through an application.
If you want irssi, I'd stick to the console version that djgoku and Jay have suggested.