Legend has it that St. Patrick drove the snakes out of Ireland, as illustrated on this prayer card:
But Gerald of Wales (late twelfth century) knew better. From the History and Topography of Ireland:
Of all kids of reptiles only those that are not harmful are found in Ireland. It has no poisonous reptiles. It has no serpents or snakes, toads or frogs, tortoises or scorpions. It has no dragons, It has, however, spiders, leeches and lizards – but they are entirely harmless.
Some indulge in the pleasant conjecture that Saint Patrick and other saints of the land purged the island of all harmful animals. But it is more probable that from the earliest times, and long before the laying of the foundations of the Faith, the island was naturally without these as well as other things.
Alas, the remarkably clear deductive power on display here is not sustained throughout the rest of the book, which repeats a lot of fanciful stories. Methinks that in this passage, Gerald was trying to undermine any claims of Irish holiness by means of justifying the English claim to the country.
The note to this passage mentions that “some modern theory on this matter maintains that the legend of the expulsion of snakes from the country is of Norse, not Irish, origin. It is based on a confusion between the Norse word for ‘toad-expeller’ (Pad-rekr) and the Irish form of Patricius (Padraig).”