I have always felt uncomfortable with the accepted explanation of natural surname evolution in this context and suspect official encouragement if not legislation to remove the Mac prefix. The history of surname development generally shows that diversity results with survival of the original name-form alongside its phonetic etc variants - the relatively sudden disappearance of Mac in the Manx native population looks like the result of a deliberate act rather than one of linguistic laziness or convenience.

Politically it would make sense as the English power-mongers took control over the island and would have wished to remove the previous (to them troublesome) Irish and Scottish influences.

On the confusion with the Norman subjugation of Ireland, you may be right, although my memory still thinks I read a comment specifically about a Manx law - I should make more notes! Ethnically, it is likely that many of the (richer) Irish fled to the Isle of Man to escape Norman persecution (and Scottish immigrants as a result of the Plantagenet wars) - perhaps the switch to English rule in the Island brought with it similar Normanesque attitudes to stamp out any remnant of irishess and scottishness, and that this resulted in the unique Manx name-forms.

Still seeking a law or Lordly edict!