I used to do this with dzen, via:

http://dzen.geekmode.org/dwiki/doku.php?id=dzen:irssi-notify

Then I started using a combination of perl scripts to detect when my screen session was detached, and if I got a new message while my screen session was detached to send an SMS to my phone through email. These days however, I'm satisfied with simply logging all my notifications in a separate split irssi window and checking them when I feel like it.