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Short answer: Because they can't swim and don't like the cold. Nope, nothing to do with St. Patrick. Ireland isn't the only island without native snakes. New Zealand doesn't have any either (although it had a snaky reptile 20 million years ago). The only snakes near Hawaii are elusive sea snakes, and the island is pretty serious about keeping out the land varieties. We hear that Greenland and Antarctica are snake-free as well. Without flippers or boats, snakes have a hard time migrating across oceans. While some islands were once part of larger land masses or connected by land bridges, cold weather and bad timing have conspired to keep snakes away from the green hills of Eire. Researchers believe that snakes evolved about 100 million years ago on the super-continent Gondwanaland, which slowly broke into Antarctica, South America, Africa, India, and Australia. That helped snakes get around the world, although at that time, Ireland was still under the sea. As the waters receded, snakes found new and exciting homes, but not in Ireland. Snakes are cold-blooded creatures, and they can't knit cozy
sweaters to survive in Ireland's chill. The country was covered by glaciers for ages, and it only thawed about 15,000 years ago. By then, no land bridges existed between England and Ireland, so any snakes pining for a Guinness were trapped in the land of fish and chips.
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