Driving the Snakes from Ireland

The legend repeated every March seventeenth is that Saint Patrick ran the snakes out of Ireland.
“But,” they say, “There are no snakes in Ireland!”
“See? He ran them all out!”
I thought that was just a silly joke, until I found where it came from.
What the good priest did was to convert the Irish to Christianity, or rather, help in the effort. He was by no means the only one to do so, there were plenty of missionaries. But he was the one who was sainted.
Before Christianity, the Irish were Celtic. The Celts were pagans, worshiping many gods, as the Hindus and Buddhists do. Their rulers were the Druids, with different Druidic orders; banking, clergy, administration, etc. The clergy were also pagan.
The ordinary Irish common-folk had found Jesus, and rebelled against their pagan Druid overlords.
It was made easier by the fact that the Druids all had snakes tattooed on their forearms.