St. Patrick was a Romano-Britain. He was kidnapped in a raid by the paganized Irish (a thing that occurred more often as the last of the legions withdrew from Britain in 410). He escaped years later, and then went back to Christianize Ireland, which he did successfully.
When the Angles, Saxons, and Jutes later took control of Britain, the place became pagan. Christianity was all but eliminated.
But then, shortly after 600, Irish monks, the spiritual descendants of St. Patrick, crossed the Irish Sea to re-convert Britain. They had great success.
This is one of those neat incidents in history: one culture evangelizing the other, then the other returning the favor many years later.
So this St. Patrick's Day, tip your green beer to the memory of St. Patrick . . . and to those swarthy holy men who have come to America from far away lands to serve our souls.