Which IRC client did you download?
Did you just use the applet linked by the site by default?
Instant Relay Chats aren't programs, they're kinda like the internet in general- things going on in magical servers far, far away. We use various programs, called IRC clients to access them like we use browsers to access web sites.
Clients connect by being told what server to look at, then what chat room. Think of IRC in general to be a skyscraper, with each server being a privately-owned floor. Each chat room is like a specific room on that floor.
I have never heard of someone getting any malicious software from the act of accessing an IRC server/room; getting some from IRC use would take either downloading a client riddled with trojans and viruses from a sketchy, unofficial source or following a bad link to a dangerous webpage.
Just to assure you of this, I have been visiting #Escapist for years, on both PC and Mac OSs, without it causing me any computer issues.
What sort of computer issues are you having? We could help you diagnose a problem.