I never get the irc I ttried a few years back going there but I'm not sure how it's different from just hanging out in the forums.
People have realtime conversations in IRC, whereas the forums sometimes go hours between posts. Plus you can steer the conversation in your direction (talkaboutgenitalsetc) whereas it's harder to do in this medium.
It's the difference between like E-MAIL and WRITING A LETTER or some shit.