I installed the IRC plugin a few days ago and configured it for my environment and customized each build's config appropriately, and the IRC bot does connect to my IRC Server (and for a bit everything worked perfectly), however, somewhere along the way the configurations for each build got lost and I don't have the options on their individual configuration screens any longer. The configuration for the IRC client under the Manage Hudson > Configure System is also missing, however, the bot does connect to the server still (but doesn't do anything because each build's configuration is missing). When looking at the Manage Hudson > Manage Plugins > Installed it does show that Hudson IRC plugin is installed.
Example project missing IRC options:
IRC Channel - Hudson is online: