The idea that the "snakes" talked about in the St. Patrick legend were actually the Pagans of Ireland is a bit of New Age conjecture that has no basis in Irish history or mythology. There were never any snakes in Ireland, ever. And the Medieval Irish came up with the story of St. Patrick driving them out to explain why that was so.
The fact of the matter is that the Christianization of Ireland was a pretty peaceful thing with no serious conflicts between the Pagans and the Christians. Paganism simply found ways to adapt to the new religion. The Pagans were never driven from Ireland. They stayed and either converted or simply quietly carried on their old beliefs in privacy.