how about picking one plugin from
http://wordpress.org/extend/plugins/search.php?q=IRC&sort=
Or look at adding webchat widget
Here's the code from webchat.freenode.net for #ilugc, some options can
be customised.
<iframe src="http://webchat.freenode.net?channels=ilugc&uio=d4"
width="647" height="400"></iframe>