In the spirit of the aterm IRIX mod article, I wanted to mention a very nice IRIX/SGI GTK theme written by Richard Offer - Buffy (shorthand for Buffed Motif, the official name for the IRIX Motif appearance). If you'd like to compile it yourself, the source code for Buffy is available from the official site at http://themes.freshmeat.net/pojects/buffy02. I also have a prepackaged tardist archive I put together a while back that will allow you to use the theme without installing a bunch of extra Gnome support packages (from SGI's freeware site), which is useful if you're low on disk space. Grab the tardist here, install using swmgr and copy the gtkrc file into your home directory (~/.gtkrc). You should be set.
For examples of Buffy GTK in action, check out some of the screenshots in the online gallery linked in the sidebar.