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Many myths surround St. Patrick. One of the best knownand most inaccurateis that Patrick drove all the snakes from Ireland into the Irish Sea, where the serpents drowned. (Some still say that is why the sea is so rough.)
But snakes have never been native to the Emerald Isle. The serpents were likely a metaphor for druidic religions, which steadily disappeared from Ireland in the centuries after St. Patrick planted the seeds of Christianity on the island.