#db is the Dark Brotherhood's main channel. Members of all of the clans can hang out here. However, you might be interested in joining the clan channel where you hold membership (or will soon hold membership).
Here is a list of clan channels of the Dark Brotherhood:
Please note that Clan Channels are not official for CoJ purposes.
You might also find these channels useful if you like multiplayer gaming:
If I wanted to join the channel called #dbgaming, all I need to do is type is:
/join #dbgaming in any of my active IRC windows.