ircu disallows usernames with more than three capitals. Since the change to PircBotX, Jenkins can't connect to IRC servers powered by ircu anymore, since the default username of a PircBotX bot is 'PircBotX', which has three capitals.
See http://code.google.com/p/pircbotx/issues/detail?id=48 for more information on the issue.
The Jenkins irc plugin can be fixed by setting the default username to 'PircBotx'. I'll create a Github pull request that does this.