If you want to auto join channels using Mirc, click on Tools, Options & Perform. In the Perform commands box type the following:
/join #ChannelName.r
If the channel happens to be passworded, and you know the password, you can join the channel by typing the following in the Performs commands box:
/join #ChannelName.r password
Click OK after each command
As a side note, you can also auto identify by typing the following in the Perform commands box:
/msg nickserv identify password
I think there's a way to auto join multiple rooms using only one command, however, I don't know what that command is. I merely add another join command on a separate line. If I'm not mistaken, if there are too many perform commands entered, you may get an error, possibly a flood error, but it's been so long I can't recall. At the moment I have 2 join commands, an identify command as well as a /list command and I've experienced no problems. Also, make sure the Enable perform on connect box is ticked.
Hope this helps.