Most terminals can be launched with a command that is executed immediately by using the -e option followed by the command.
gnome-terminal -e irssi
launches a terminal with irssi running in it.
This can be useful if you want to save yourself having to run irssi each time you log in. In Gnome you use the application gnome-session-properties to
schedule commands to run at login.
Add the command
gnome-terminal -e irssi
and a title and description.
Now on login you will have a terminal running with irssi.
If you want to have the window maximized, you can append the option --maximize.
gnome-terminal -e irssi --maximize