Carl McColman has an excellent blogpost reflecting on St Patrick's Day and why Pagans don't regard it as a celebration. It's well worth a read.
That old chestnut about Patrick driving the snakes out of Ireland does, as he suggests, seem a somewhat tenuous quote on which to base an accusation that he drove paganism out of Ireland. As he says, it went underground, and was most likely driven there by the entire power structure of the church and the higher echelons of society who had embraced Christianity because everyone else in Europe was doing it.
But it's a great shame that Christianity decided to abolish the old deities (or at least, the ones it couldn't assimilate as saints). When Buddhism arrived in new countries, it simply declared deities irrelevant to its message and let people carry on honouring them.
I do agree with his conclusion that everyone should work together for the eradication of slavery and the promotion of social justice.