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It is theorized that Saint Brendan, called "the Navigator" and "the Voyager" is the first European to discover the America's. Going forward on this theory, what if, when he discovered the land, he was (for the sake of the what if) experienced a vision that it was his duty to bring Christianity to this new land, and thus with his fellow adventurers sets up a colony where they land, then goes back to Ireland for more volunteers.
For my own part (due to the times this went on in) I don't think he'd get many, but what effect might there be if a permanent colony was set up by the Irish Missionaries?