While there is no shortage of general purpose chat clients available for OS X, when it comes to IRC clients, software is a little more thin on the ground, particularly quality applications. Colloquy is an advanced IRC, SILC and ICB client that is open source and therefore available completely free of charge.
Colloquy includes everything you would expect to find in a chat client, from buddy lists and emoticons to the ability to work in single or multi-window mode. The application enables you to chat on more that one server at the same time, and when transferring files between users, transfer resumption is available in case of interruption.
One of the key advantages of working with Colloquy over other similar clients is the way in which it can be customised and extended. A massive range of plugins are available to add all manner of new features and options to the application.
AppleScript support means that common tasks can be automated and the scripting process is kept as simple as possible. Colloquy is one of few IRC clients that retains OS X's native look so you'll feel immediately at home as you chat with others from around the world.
IRC is an old protocol, but it's here to stay and Colloquy will enable you to get the most from it.