Let's be very clear - St Patrick was no friend of Pagans. However he is in no sense responsible for a genocide of Celts, Pagans or Druids in Ireland. Since the bad folkore that circulates on the internet has trouble distinguishing between those three terms, let’s start there.
Celts refers to a language and cultural family that was prominent throughout Europe between c. 800 bce and the conquest of most of their territory by Rome at the turn of the first millennium ce. The Irish of St Patrick’s day were Celtic-language-speaking people, living a Celtic-style culture. They remained so after the conversion to Christianity, save for the radical change in religion.
Druids were the priests, judges, healers, and magicians of the Celtic tribes. They were not themselves a people or tribe – there was no ‘druid people’. Rather Druid was a profession among the Celtic peoples.
“Pagans” is what the incoming Christians later called the religions of the traditional peoples of Europe. Characterized by land and nature-centered worship, polytheism and ancestral heritage, it became a term for “the Enemy” as the Roman Church gained power. Celts kept Pagan ways, the Druids were the Celtic priests of those ways.
Pagans being readers, most of us know the outline of Patrick’s legend. A British Celt, he is taken by raiders and enslaved in Ireland. Escaping, he returns to Britain. Converted either miraculously on an Irish hillside, or culturally in post-Roman Britain he undertakes the priesthood. His visions call him back to Ireland, and his mission becomes the stuff of legend.
Legend is the right term for most of the story of Patrick in Ireland. Druids managed the Irish religious life. Since St Patrick made it his job to “bring souls to Christ” he would have found himself preaching in opposition to the Pagan ways, and in opposition to Druids. He (or “God”) is recorded as defeating and destroying various Druids in what amount to magical battles. These are the fantasies of the Church novelists who write the “Lives of Saints”. In some stories St Patrick takes the place of Pagan Gods, such as the famous tale of the destruction of Cromm Cruach
Snakes are not a metaphor for Pagans in the famous story. The snake legend occurs very late, and is not part of the original Patrick tales. It was copied from other saint's lives to add another cool story to Patrick's tale (See Morgan Daimler's article, below).
There was never a holocaust of Paganism in Ireland – at least not in the early stages. At St Patrick’s death a few kings had been converted, and the first monastic houses had been founded. Those houses, no doubt informed by the Celtic tradition of scholarship, later helped restore Christianity to Post-Roman, Anglo-Saxon England. The Church grew slowly over several centuries, and no Christian missionary was ever martyred by the Pagan Irish. Even in the 7th century (hundreds of years after Patrick) the compilers of law saw need to discuss the place of Druids in the system.
Here’s the thing – I love St Patrick’s day. Don’t let any modern Pagan tell you that sobriety and modesty is the traditional way to keep Pagan customs! I enjoy the celebration of Irish heritage, culture and struggle here in the US, and the chance to tell a few truly old stories, some years. However it seems just kind of… lame to spend emotional energy “Hating the Romans” here in our age, and St P just wasn't particularly a villain.
Here are a few links that do a better job on the scholastics of this than I mean to do this morning.