Methinks the "IRU LEFT" QRH is wrong.
Not sure, McHale. QRHs and FCOMs are usually the latest and greatest. My engineering notes are probably as old as the aircraft themselves. The QRH seems very specific in the wording (i.e. it actually reads "Body Gear Steering not available with IRS LEFT
message". This would allow operation with the IRUs off. I'm not sure if the message is displayed with the IRUs in ATTitude. If it's only the Left, then maybe Captain's IRU Source would allow Right or even Centre.
Unfortunately, this is not easy to test in real life or in the sim.