Your key words here are "update the IRS position". It does not up date the IRS. I'm amazed at how many people hold this misconception. The GPS signal is used to upgrade the FMC position, not modify the IRS position. As you have stated, the FMC (FMS) takes all this information, weighs it for logic, and then applies it to the FMS position. Typically you can look and see what nav reference is doing the heavy lifting to come up with your position.
If you were flying in a non-GPS equipped aircraft, there would be a limit on how many hours you could operate without some sort of FMS update, typically a VOR, before you could continue. In the GPS equipped aircraft this update is faily constant except at very high latitudes, thus on the B777 the time of operation is limited to 24 hours and not likely to effect any stage lengths in todays operations.