Yeah, its possible, but you wouldn't be able to do it on the RTB IRC that's built into RTB. The IRC wasn't made by Ephialtes, he just made the channel #RTB on irc.centralchat.net
You can make a private IRC Channel if you use a different IRC Connector, like pidgin. You used to be able to make a private IRC Channel with the chat command /join channelname but Ephialtes removed that command from the Blockland IRC.
Bottom line is: You can make one with something besides blockland IRC like pidgin or trillian (i prefer trillian). Oh, and no one can join the irc channel unless they use pidgin or trillian or something like that.
Aren't we talking about the RTB's IRC client?
Yes, we are. We can no longer make private IRC Channels nor join other ones with the /join channelname command because Ephialtes removed that from RTB's IRC client.