WordPress.org

Author: Sudar

Well WP-IRC Plugin (for lack of a better name ;) ) can fetch the number of users online in any IRC channel and can be scheduled to auto refresh it every x.minutes. The count thus fetched can be displayed in the sidebar of your blog using a sidebar widget provided by the Plugin.

You can also allow users to subscribe to email alerts like, “alert me after x number of people has come online”.

More information about the Plugin can be found in the Plugin's homepage