irc: is used to refer to IRC channels and nicks.
"In trying to figure out why, I looked in the help menu which led me to launchpad which links to IRC channels; those irc: links Just Work in Ubuntu. Yay! Except that xchat didn't realize that I was already logged into the Freenode network. Odd." -- [DanC Jun 2007
There has been at least a little progress toward standardization: 1996-08-26 draft by Mirashi .
Real-Time Resources in the Web: IRC, Telephone, Instant Messaging DRAFT started 7 Jan 2001 by DanConnolly
See also other UriSchemes.