Irssi is made with Qwerty in mind. If you happen to have more than 10 windows open you usually navigate with Alt-Q for 11th window, Alt-W for 12th, Alt-E 13th.. etc.

With Colemak it's fine till the 12th window.

Beyond that paste this in irssi:

/alias colemak bind meta-f change_window 13;bind meta-p change_window 14;bind meta-g change_window 15;bind meta-j change_window 16;bind meta-l change_window 17;bind meta-u change_window 18;bind meta-y change_window 19
also
/bind meta-; change_window 20

Now just type /colemak in irssi and the entire top row of the keyboard will use the colemak layout. I've added the extra line /bind meta-; change_window 20 because the ; symbol is part of the alias syntax and thus needs to be added separately.

To get back to Qwerty (not recommended though :) )

/alias qwerty bind meta-e change_window 13;bind meta-r change_window 14;bind meta-t change_window 15;bind meta-y change_window 16;bind meta-u change_window 17;bind meta-i change_window 18;bind meta-o change_window 19; bind meta-p change_window 20

And after that just /qwerty

Now with the two quick aliases you can switch back an forward as you wish.

NJoy chatting ;)

Edit: Fixed the 20th window issue