On the internet there are many irssi-notify.pl/notify.pl/growl.pl/growl-notify.pl scripts that just don’t work.

This is an irssi script that works brilliantly on OS X 10.7

https://github.com/sorin-ionescu/irssi-growl

I upgraded my perl CPAN Bundle: sudo perl -MCPAN -e 'install Bundle::CPAN'

Then installed the two required modules: Growl::GNTP and IO::Socket::PortState

Copied the png and script into place and voila!