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.