ZNC is an IRC bouncer with many advanced features such as a built-in web interface, persistent connection (detaching), multiple users, per channel playback buffer, SSL, IPv6, transparent DCC bouncing, and c++ module support, to name a few.
Traditionally, IRC clients on the Mac have been anything but glamorous. Colloquy is an advanced IRC client which aims to fill this void. By adhering to Mac OS X interface conventions, Colloquy has the look and feel of a quality Mac application.