The Unity IRC channel is #otee-unity on freenode.
If you are new to IRC, you will need an IRC client. Here are some free Mac friendly ones:
The server name is irc.freenode.net and the channel name is #otee-unity.
Because it's stable, economical in terms of resources, and provides moderation if needed.
If you need help connecting, there is a thread on the OTEE Unity forum about IRC here.