It is not always convenient to use ssh port forwarding to workaround an IRC server that only allows connections using client SSL certificate. I don't believe this feature is currently available in Colloquy, and therefore request client SSL certificate support.
Regarding possible OS X built-in support... In Leopard the Keychain Access.app allows right-clicking a certificate and "New Certificate Preference..." to create an Identity Preference item in the Keychain. This allows the user to force Safari to use any client certificate with a particular website. That said this feature doesn't appear to be fully implemented in Leopard as I haven't figured out how to setup a wildcard to cover an entire website, and I haven't found much on the Internet although I haven't checked Apple's Developer documentation yet.
