Pidgin is a cross platform instant messaging client that has multiple chat protocols including IRC. It has support for almost every other chat service available.
Pro Support for a wide range of additional chat services
Pidgin not only supports IRC, it also supports plenty of other services such as AIM, Google Talk, MSN, ICQ and plenty of others. This way, you can have all of your conversations in the same app and won't have to deal with multiple chat windows being open.