The mircryption addon is made for the mirc and xchat clients (open source, mac support).
The Fish addon (not written by us) is compatible with mircryption and is available for mirc, xchat, and irssi.
The open source KVirc client (free open source) has recently added support for mircryption-compatible encryption/decryption, and it even supports the new cbc modes. It is cross platform for Linux, Mac, Windows, FreeBSD.
We have recently added mircryption support to psybnc (see link in sidebar), which when run locally, will allow you to use any client to speak encrypted.
The open souce, cross-platform java jIRCii client now supports mircryption/fish, thanks to butane (jIRCii coder) and k2r (java mircryption coder).
The first mircryption/fish compatible handheld phone/pda irc implementation is now available for symbian/uiq smartphones: Quirc.
New *nix irc client adding support for mircryption/fish: Konversation.
The XIrc IRC client for MAC OSX -- now supports mircryption (for Mac also see xchat and kvirc above).
New script (blowssi) for irssi encrypting/decrypting which included support for advanced cbc mode; discuss on the dc forum here.