Spodman, as far as I know, IRS cannot be updated once 'aligned' without a ground re-alignment. (I am some years behind modern technology, so please someone correct me if I'm wrong).
What IS updated is the navigation system position that the IRS feeds. Any IRS errors that are there after alignment remain for the whole flight (or worsen).
IE If you have a moving map display atached, that 'position' can be updated, if it is an FMC, that can be (constantly) kept updated, but once the update facility (visual, DME, GPS, whatever) is no longer available, the kit reverts to IRS position only navigation. This function CAN be 'trained' to learn whatever errors it has acquired, while updating is available.