Saint Patrick's achievements were many and real, but driving snakes out of Ireland was not one of them. Solinus noted the island's snakelessness long before Patrick arrived in Ireland in the 5th century. The historian Bede, writing in the 8th century, confirms the absence of snakes on Irish soil, and makes no mention of Patrick. Bede tells us that when snakes were transported to Ireland on ships, they invariably expired when the ship reached the midpoint of the Irish Sea -- exterminated, presumably, by their first whiff of Irish air. Bede also claims that people in Britain suffering from the bite of vipers could be cured by drinking water in which the scrapings of Irish books had been steeped.
None of the early biographers of Patrick mention the episode of the snakes. So if not Patrick, then who? Or what?
The answer is both topological and climatological. During the most recent ice age, the flora and fauna of the British Isles were pushed south by the advancing glaciers. Then, about ten thousand years ago, as the ice caps on the northern continents began to recede, the plants and animals came creeping back, first to Britain, then to Ireland. For a time, when great quantities of frozen water were still piled on the continents, Britain and Ireland were connected to the rest of Europe by dry land. Across these natural bridges the creatures moved. But as the ice continued to melt, the level of the seas rose, and the lowest lying parts of the European continent were flooded. Britain and Ireland became islands. The snake, deliberate creature that it is, made its way successfully from France to Britain while the present channel was still dry, but its migration further west was cut short by the rising waters of the Irish Sea. The quicker lizard established itself as the only reptile on Irish soil.
But the mystery is not fully resolved. It has been ten thousand years since the end of the last ice age, and it seems surprising that in all of that time snakes have not somehow managed to reestablish themselves in Ireland. Zoologists have debated whether there is some property of the Irish climate or soil that has prevented the successful reentry of snakes into Ireland. As far as I know the issue is unresolved.
The Irish naturalist Robert Lloyd Praeger tells of a test of the compatibility of snakes and the Irish environment that had a curious outcome. In 1831 a snake was found at Milecross in County Down. The idea of a "rale live sarpint" having been found so near to Saint Patrick's burial place created a sensation among the country people. One clergyman preached a sermon citing the snake as a portent of the coming Millennium.
It turned out that the unfortunate reptile was one of half-a-dozen harmless English garden snakes that had been purchased in Covent Garden Market in London by a gentleman who wished to learn why the Irish environment was destructive to snakes. He turned the reptiles loose into his garden near Milecross and they promptly escaped into the countryside. Four were eventually killed.
The fate of the other two Milecross snakes was never determined. They did not, it can be confidently asserted, manage to establish themselves as permanent residents of Ireland. Whether the reason was bad luck, some inclement quality of the Irish climate or soil, or the baneful effects of Patrick's supposed malediction I will leave for you to decide.