Since pales in England go back quite a long way (in print 14th c.), beyond the pale" may go back before before its application to the Dublin area that remained under English law. One reference from 1347 refers to pales bounding a Seignur's demeigne (OED). The English 'pale' existed in the Dublin area by 1400, so McGrath could be correct.
There was also the English pale in France.
In any case, it is interesting that the phrase has persisted for such a long time.