MacIRC.co.uk

Another new client has recently been released for OS X, entitled Zirc. According to the client home page, it offers the following:

- Threaded DCC Chats and Transfers
- Nick completion and history
- mIRC Colour compatibility
- Multiple Server capability
- Automatic Nickserv Identification
- Fully Customisable colour schemes

Zirc is not free, and will set you back $19.95 for a license.

Zirc Home Page