Patrick was an escaped slave that converted to Christianity and traveled to Ireland to spread the gospel to the heathen therein.
Patrick was a Romano-Briton. His place of birth is not clear, some suggest Cumbria while others indicate Northamptonshire. Whatever the case, his father Calpurnius was a decurion and a deacon – he himself the son of a Romano-British priest Potitus. As a young teenager, Patrick was captured by Irish slave-raiders and taken to Ireland, where, as the story goes, he regained his faith, so to speak. He escaped back to Britain and then decided to go back to Ireland to spread the word. I don’t think Patrick converted to Christianity, he was a baptised Christian, he just became filled with piety and spirit because of his admittedly traumatic experience.
Nevertheless, the whole story, dare I say mythology, of Saint Patrick is so fraught with problems from a historiographical point of view, that a very large pinch of salt should be taken with any of it, for example, the business of the snakes. A lot of what is purported to be “known” about Saint Patrick’s life and deeds comes from later hagiographers who, naturally, had their own agenda when writing all of this down. Another difficulty arises in that Patrick was by no means the first missionary to the Irish, Palladius, a Romano-Gaul, was actually the first bishop to the Irish, preceding Patrick, not to mention the so-called “pre-Patrician” saints of Ireland. In fact, it may be the case that the whole hagiography of Patrick is a pseudo-biographical/hagiographical blend of the lives and deeds of a number of saints in Ireland – especially Palladius.