Thank you too, CaptainSandL. In the occurrence I described, the left IRS first started to drift; then after several minutes it failed altogether: the IRS FAULT light came on. Hence my strong belief that the mapshift was directly related to the IRS failure.
What's puzzling is that in this incident, after the left IRS had been switched off according to the IRS FAULT procedure, the FMC position still remained incorrect, despite two (retrofitted) GPS receivers being available (supposedly not only as stand-alone devices or for EGPWS purposes, but actually integrated in the FMC navigation logic).