Why not just unblock port 6667 for outgoing connections? Since existing mechanisms (see http://it.jhu.edu/restricted/infosec/blocklist.html ) automatically detect and block hosts with malware which uses IRC, clearly, blocking outgoing traffic on port 6667 only blocks legitimate traffic because hosts sending traffic from malware are already automatically blocked by the mechanism seen at http://it.jhu.edu/restricted/infosec/blocklist.html .
Keep in mind IRC has many legitimate uses. For example, irc.freenode.net has many IRC channels (chat rooms) for help with open source software. It also educational channels (chat rooms) from organizations like NASA. Additionally, irc.mozilla.org has channels (chat rooms) for help with Firefox.
I would be happy to discuss this issue further.
Thank you.