When I was in Ireland last month I took the Viking Splash Tour in Dublin with my daughter and her Irish cousins. According to my tour guide, the Vikings did not have hats with horns. The Celts and later, Catholic priests, would draw them with horns coming out of their heads to make them seem more demonic.
Well, the Celts and the Catholics had good reason to think the Vikings were pretty nasty!
As I recall (and I can’t remember the source), the legend that Viking helmets had horns started in the 19th century when artists put the kind of horned helmets worn by ancient Germans on Vikings in their artwork.
Ireland ping! Ufda!