For this kind of question, the best way, if you found nothing in the libraries page, is to type something like java irc in Google or similar, and find libraries like The Martyr Java IRC Library...

Well, I never used IRC yet... It is on my TODO list to try that but never found time for that. Not sure how to use such library without being noisy in a channel.

Anyway, you don't need to know Java to get it working. You should have a .jar file with the library. Put it in a code folder in your sketch folder (next to the .pde file) and you should be able to import its package(s) and use them as shown in the examples probably given with the library. Just don't copy them slavishly, use instead the content of the main() method which should be there.