He's postulating that prior to the vikings,Iceland,Greenland,the Maritime Provinces,and parts of the far northeastern United States were settled by a people of Celtic-and Christian-background. Mowatt makes it clear that a long voyage(i.e. Europe to North America)in an open boat without high tech gear of navigational aids isn't nearly as impossible as a lot of people like to think it is.(apologies for the off-topic post,btw)
What was his dating? Do you recall if reconciles the Celts not being Christians, at least until the time of Christ of course? Some of the earliest Christians were in fact Celts, those in Galatia (Turkey) and other near areas to whom the Apostles were sent.