Well, it’s documented that Leif Ericson established a Norse colony on the Canadian coast. It would not be out of the question that he would have told some Irish of the colony, maybe had some come with him, and perhaps some colonists stayed behind and moved south as the Medieval Warm Period ended.
The Vikings were ‘in’ Ireland, too. The Dublin-area was a Viking kingdom, IIRC. I’m sure that the information of lands further west was common knowledge in Ireland around the year 1,000 A.D.