Did you know that not only are there no snakes in Ireland, there never were? Neither are there moles, several species of bats, many bird species, and innumerable insect species. Wanna know why? It’s because Ireland is actually fairly small. One law of ecology is the more land, the more species, the more diversity.
Present day Ireland was assembled largely after the glaciers of the Last Ice Age retreated. So that means all the wildlife that is there got there by migration. So anybody with wings or who could swim, could take a shot at the Emerald Isle. Which pretty much leaves out snakes. Hard to do the back stroke with no arms. Or legs.
The legend about Saint Patrick probably came about as Christianity made its way pub-ward, pushing out the early Celtic pagan beliefs. And one of those beliefs was a matriarchal religion, and the snake is a representation of the Triple Goddess in the pre-Celtic culture. When the patriarchal and warrior-like people became dominant, the symbol of the serpent was crushed. So Saint Patrick served also as a symbol of the crushing of the serpent, as the Druids and their culture was crushed.
The Druids associated the serpent with the sun. They represented the creation and the universe by a serpent in a circle, sometimes by an egg (the cosmic egg) coming out of the mouth of the serpent. You know who else did that? The Phoenicians and the Egyptians.
Saint Patrick was not a unique reptile repeller. There is a whole history of heroes defeating serpents, snakes and dragons, starting with the Greek, Herakles. We just cleaned up the details and assigned the role to poor Patrick, who was just trying to do his job in the fifth century, in this clash of the two cultures.