The IRU isnt exactly a navigation sensor: it is more like an error reporting unit.
When you start in the morning, you tell it where it is. Either by telling it the gate position or by giving it the GPS position.
What it then tells you is where it isn't, as it measures accelerations away from that known position.
Yes and no.
Position is only one output from an IRS.
When position output is not valid it may still output accelerations or track and groundspeed.
The FMS can use these outputs to navigate in dead reckoning mode. In this case it is sensing change in position so I guess it is a position sensor. Or at least thats how the manufacturers see it. I'm not going to argue with them.