In the SGI slang, the word "Overlay" just means "update".
Overlays are only available for IRIX 6.5.x, and you cannot update
a pre-6.5 installation to 6.5.x, even not by trying to install
You can download overlays directly from SGIs
website, but beware that you won't get the newest version
unless you have a support contract. Also note that if you download
overlays, you cannot create a bootable CD without some big efforts.
But you can still use the downloaded version for updating a working
IRIX 6.5.x system or you can even use the downloaded version for
booting over a network.