I would try the following:
Bring up the mIRC options dialog by pressing Alt-O on your keyboard. When the "mIRC Options" dialog is showing, just press O on your keyboard. This will highlight Options and will show a group of buttons, one of which is labelled "Perfom". Click the "Perform" button. This will bring up the "Perform" dialog. In the "Perform commands:" area, type join #yourchannel yourpassword.
For example, if your channel was #ocelots, and your password was cats, you would type join #ocelots cats.
Don't forget to select or add the appropriate network while you have the "Perform" dialog open.