First, in Terminal preferences, make sure that the default login shell option is checked, and that "open a saved .term file" is empty/unchecked.
If that looks clean, look for files named .profile and .bash_profile (there could be one or both) in your home directory. Make sure that there isn't an irssi command in either one. (You probably want to remove the offending command, if any, with a text editor, but keep the files around)
Do those places turn up anything?