I'm not sure if it's supposed to be because someone is common or because they are "Celtic" that it is assumed they couldn't know the basics of their religion, or even be able to say the Rosary.

Actually, from what I've been reading recently ('Visions of Isobel Gowdie') it would seem that the bulk of people (the bulk of people being fairly poorly educated) and especially Scots in the highlands, wouldn't have had much if any appreciation of the complexities of theological doctrine. Which wouldn't surprise me all that much to be honest.

Where did it say in that article that the average Celt or Scot couldn't say their Rosary? Theological doctrine is one thing however, and I imagine anyone of whatever background can manage mindless repetition.