are you not thinking of Ireland where the adoption of Gaelic names by the Norman lords was I think actively discouraged.
As far as I know there was never any Manx law - the various authors on the Etymology of names look on the change as fairly natural given the effective decoupling of Mann from Ireland - possibly hastened by English scribes who wrote down the name as they heard it though the survival of the ine (daughter) form of some names indicated that the mac/ine was understood and used by many.