Yes, we do have GPS navigation; it was retrofitted several years ago, and my degradation list comes straight from our 733/734 FCOM (we also operate NG's, so I double-checked today to be sure I had used the correct manual). Your comments are interesting, though, because I wouldn't rule out a discrepancy between the operation manual and the actual retrofit status.
But even without GPS, I wouldn't have expected an FMC map-shift to occur with one faulty IRS: the degradation rules continue with DME/DME, VOR/DME, LOC/DME, LOC, and I am pretty sure DME/DME or at least VOR/DME would have been available at the time of occurrence. (A faulty DME update is possible, but highly unlikely, since the FMC position shift occurred at the same moment the IRU started drifting.)
Now, this map-shift could very well be normal system behaviour, but based on the FCOM only I wouldn't have thought so.
