If you're on Linux, you can find IRC client's in the Ubuntu Software Center, just search "IRC" or "IRC Client". But Karol is correct, there are a lot of IRC client's out there
--Edit-- If you're using a Linux Based-OS besides Ubuntu, you'd also want to try and use the Terminal.
mIRC is cool using it has many advantages like alias for controling your chat room and the advantage to help people with your alias and lots more if you run a irc bot mIRC you can't use the second choise is XChat and that is cool but i would not say it is the best