The term has been used loosely, but I was always told it meant dark haired and reddish skin. It has been erroneously linked to the defeat of the Spanish Armada and shipwrecked spanish landing in Ireland, probably to excuse the dark characteristics. The fact is the Celts inhabited France, Spain, Germany and most of central europe. My guess is that the so called Black Irish are probably more common. It is the fair skinned ,blond Irish that may be of foreign origin...viking or other.