I'm not familiar with your type but in general an FMS system will blend position sensors to compute the FMS position. The accuracy of the position ANP(EPU) depends on the sensors available and the priority that the FMS uses. If RAIM is yes then the FMS will probably use 100% GPS position so the accuracy of the IRS position is irrelevant.
The normal sensor priority would be:
GPS
DME/DME
VOR/DME
IRS
as this reflects the accuracy of the various nav systems.
Although you may have a good IRS even the best can drift at 2 nm/h.
Some systems monitor irs error when other sensors are being used so that when the irs is the only sensor it will keep the last known offset from irs position and not jump(map shift) to the raw irs position.