With so many different options and retrofits etc. it's very difficult to give definitive answers, but isn't the FMC position a calculated compromise between the 2 IRS positions and radio and other positions? it is on the basic 300-400 kit.
If say the radio position is temporarily lost the FMC position continues to move relative to the IRS positions rather than give a huge instantaneous shift.
If that is the case then if one IRS goes faulty and starts drifting about then the calculated position could also wander about presumably causing a shift.