Today, I've merged my erc notifications module inside Emacs trunk. This way, it will be available for everybody in Emacs 24.2.
This module allows you to be notified via
notifications-notify (the Emacs implementation of the Freedesktop desktop notifications) on private message received on IRC, or when your nickname is mentioned on a channel.
To enable it in Emacs > 24.1, you just have to add the following line to your configuration file:
(add-to-list 'erc-modules 'notifications)
If you want to download it and use it stand-alone for a previous Emacs version, you can fetch the latest file revision and load it before adding the previously given line.