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”.

Screenshot

Links

The Plugin is available on the WordPress Plugin directory. Support is available at Plugin’s home page or at the author’s original announcing post.

Your feedback’s are welcome!