So the Irish were not cruel to the Romans before the Vikings arrived. Sounds like romatic BS to me.
As for the Vikings being cruel; about as cruel as the Saxons, Normans (Frenchified Vikings), Vandals, Goths, Visigoths, and all the other humpty-dumpties that came along. In ancient Ireland one of the great pastimes was the raiding for cattle and wives. Our family takes it’s name from the Viking Hrothrekr. We’re as Irish as they come. We stole cattle and women from the Gaels and they from us. It wasn’t long until the bloodlines were so mixed that the only Viking left was in some root words and names. Our family was deposed by William of Orange after the Battle of the Boyne. Our land and wealth forfeit but not our titles or honors. And so it was, defeated by an English come Dutch king. It is the history of the world. Families, peoples, nations raise and fall. Bloodlines mix. None of us are really what we quite romantically envision. I think I’ll have some more of that liquid sunshine and get off my soapbox.
Yes; the teenager who would eventually become Saint Patrick was actually a British adolescent fishing on the shores of Cornwall, from whence he was kidnapped and enslaved by Irish pirates in the 800's A.D.