The original version of this can actually be found on http://wouter.coekaerts.be/irssi/. This script enables the mouse scroll wheel to scroll the channel content as well as supporting some gesture recognition to do stuff.
The problem with the mouse scripts is, that since they enable XTerm mouse reporting, but Irssi does not actually support this in any way, nor is it possible through scripts to simulate copying and pasting of text, that it is difficult to copy like e.g. quotes or URIs out of the Irssi terminal window.
This version is enhanced by Soliton to provide easy means to disable the XTerm mouse reporting mode, in order to allow for easier access to terminal copy&paste behaviour.