Another IRIX game worth mentioning is Racer, currently at version 0.4.9. Racer is a free car simulation project offering real car physics as well as the ability to easily build custom vehicles and tracks. This focus on realism makes Racer more suited to hardcore sim fans than casual arcade gamers.
You can download the IRIX build here and the MacOS X build here. Note that unlike games such as Quake 2 which can utilize software rendering if a system doesn't have texturing in hardware, Racer absolutely requires texture support.
Racer enjoys a very active user community which contributes tons of tracks, cars, and other goodies. The Racer forum at Race Sim Central is a great place to start.