Legend has it that St. Patrick drove the snakes out of Ireland in the 5th Century. While it is true that Ireland has no snakes it is for geographical, not human, reasons. When the snake evolved about 100 million years ago, during the late Cretaceous period, Ireland was completed submerged under water. During the Cenozoic Era, about 65 million years ago, the world started to dry out and vast grasslands dominated the Northern Hemisphere.
Predecessors to the cobras and pythons first appeared 50 million years ago while modern day cobras and pythons appeared 25 million years ago. Today, snakes are found everywhere but Ireland, New Zealand, Greenland, Iceland and Antarctica. Serpents haven't figured out how to migrate across the ocean to a new home. While land bridges formed over past millennia, snakes would have frozen during the Ice Age.
How did the myth of St. Patrick and the snakes evolve? In Christian cultures, snakes have always symbolized paganism starting with the serpent that tempted Eve in the Garden of Eden. Therefore, St. Patrick, who helped convert many pagans to Christianity in Ireland, would be a likely candidate to banish the snakes.