Fifth Century Legend has St.Patrick standing on a hill using a staff to herd the slithering cold-blooded creatures into the sea.
Cold-blooded? They become the temperature of their surroundings.
Why no Snakes in Ireland? The Ice Age. More than twenty times Glaciers advanced and retreated on Ireland and only let it thaw out 15,000 years ago. Those snakes reaching Ireland would have become Popsicles. Hence, Ireland has 12 miles of icy-cold water in the Northern Channel separating it from Scotland where several snake species abide today.
There are no snakes in Ireland for the simple reason that they can't get there.
Luck of The Irish!