Hello Alex Whittingham
It is also my understanding that an IRS is not NAV updatable in flight. After the POS parking alignment, it "only" delivers its calculations to the FMC in a continuous manner without being updated. It is the FMC - downstream the IRS' - that refines the navigation by using other external positions sources that it processes through a mathematical model (Kalman Filter) to compute what is supposed to be the most likely position with some degree of trust showed by the ANP (actual navigation performance).
My question is more about on what exact criteria an IRS may be deemed unreliable by the FMC, and what is the order of size of the drift magnitude that would send a "fault" Master Caution to the Mide Selector Unit...
Cheers