I don't know if specific IRC users have been sued for it before, but there's certainly no reason they couldn't be. At the start of this year several IRC servers were contacted by the MPAA regarding filesharing, as a result many IRC servers either banned filesharing altogether or specifically changed their Acceptable Use Policy to state that they would supply any information on users regarding illegal filesharing to authorities if it was requested. Even for those that haven't banned filesharing and don't specifically state those things in their AUP, if they didn't comply to such a 'request' (read: demand) they would probably become targets themselves for legal action. In other words almost any IRC server will sell you out in a heartbeat if push comes to shove. Of course that's if information from the IRC server was even required. I think there's a good chance if you were using a server without hostmasking then there would be nothing needed from the IRC server at all.
Short answer: Yes, you could be sued.
