To use an IRIX system without a monitor and keyboard attached (i.e. "headless") the NVRAM must be set so that this configuration is recognized and not interpreted as an error. When the SGI machine is set to boot in this way, the only ways to connect to it are through a serial console or a valid network connection. Even without its own monitor, it is still possible to use the machine as a headless desktop.
The key variable is "console". To set this from the PROM monitor:
>> setenv console d
To set this from a root prompt:
# nvram console d
To boot normally again, change this value back to "g".