The moment GPS updates an IRS position it is no longer an independent nav sensor!
The FMS manages the aircraft position based on all available nav sensors with weighting for the most accurate.
The FMS position will therefore be a blended position based on either GPS or IRS or an other depending on where you are and what's available.
Some FMS monitor the IRS error from to GPS position and apply this correction to the raw IRS position if GPS is lost. Thus the FMS position is a corrected IRS position but the IRS position remains uncorrected.