If St. Patrick didn't chase the snakes out of Ireland, what did?
Dear Straight Dope:
Early Christian myth suggests that St. Patrick chased the snakes out of Ireland. This sounds like a load of rubbish to me but the fact remains that there are no species of snakes native to Ireland. Why is this?
Snakes in Ireland were wiped out not by St. Patrick, but by the last ice age. Up until roughly 10,000 years ago the British Isles, along with most of the rest of northern Europe, was covered by icecaps and glaciers, not the most snake-friendly of environments. Both Ireland and Great Britain were part of the continent then--sea level was lower since so much of the Earth's water was locked up as ice. Snakes survived in southern Europe, where conditions were warmer. Once the climate improved, snakes were able to recolonize northern Europe, but didn't manage to reach Ireland before rising ocean water caused by melting ice cut them off by forming the Irish sea--snakes don't cross water very well. Only three species of snakes were even able to reach Great Britain--the grass snake, smooth snake, and adder. They either colonized it before the English Channel formed, or perhaps were somehow able to cross it afterward.
The British Isles as a whole are pretty poor in reptiles and amphibians in general. Besides the three snakes, Great Britain has only three lizard species, one frog, two toads, and three newts. Ireland does even worse, with no snakes, one lizard, one frog, one toad, and one newt, all of them species that also occur in Great Britain. And the frog may have been introduced by humans.
Many have explained the legend about St. Patrick and the snakes as a metaphor for his success in converting the pagan Celts to Christianity. Snake imagery has been important in many ancient religions, often as a symbol of rejuvenation or rebirth due to the snake's habit of shedding its skin. In Ireland, the snake symbol was associated with some Celtic goddesses, and also with the cult of Crom Cruaich, which demanded human sacrifice to a serpent deity. Patrick did not drive snakes themselves out of Ireland, but rather these Celtic snake spirits. How did the Irish Celts come up with symbolic snakes if they'd never seen real ones? First of all, their ancestors, and their religion, had come from the European mainland where snakes were plentiful, and second, they were in frequent contact with areas that had snakes, most obviously Britain, where the adder was capable of memorable bites. It's not surprising that such enigmatic creatures would be preserved in mythology even if they weren't present physically.