I'm using irssi on Gnome 3, but I do use KDE 4 sometimes. I like irssi very much but I miss any notification feature. I don't use irssi over ssh.
There seems to be a couple kicking about:
They're just Perl scripts you drop into
http://code.google.com/p/irssi-libnotify/ works great for me.
If you want it to notify all messages instead of just hilighted ones remove