In iTerm2 select Preferences -> Profiles -> Default -> Keys -> Add key mappings:
Keyboard Shortcut | Action | Value |
Cmd+q | send escape sequence | Esc+ q |
Cmd+w | send escape sequence | Esc+ w |
Cmd+0 | send escape sequence | Esc+ 0 |
Cmd+1 | send escape sequence | Esc+ 1 |
Cmd+2 | send escape sequence | Esc+ 2 |
Cmd+3 | send escape sequence | Esc+ 3 |
Cmd+4 | send escape sequence | Esc+ 4 |
Cmd+5 | send escape sequence | Esc+ 5 |
Cmd+6 | send escape sequence | Esc+ 6 |
Cmd+7 | send escape sequence | Esc+ 7 |
Cmd+8 | send escape sequence | Esc+ 8 |
Cmd+9 | send escape sequence | Esc+ 9 |
After this Applekey+numbers, Applekey+q and Applekey+w change the window in Irssi.
Also recommended: KeyRemap4MacBook: Command_R to Option_R.