The saint we celebrate on March 17th is very different from the man who became St. Patrick. The historical Patrick didn't drive the snakes out of Ireland, did not use the shamrock to teach about the Trinity, and wasn't even born in Ireland. However, what he did was quite remarkable. It stemmed from a vision he had after escaping slavery in Ireland, that told him he needed to return and minister to the Irish people.
He spent his last 30 years there, baptizing pagans, ordaining priests, and founding churches and monasteries. His persuasive powers must have been astounding: Ireland fully converted to Christianity within 200 years and was the only country in Europe to Christianize peacefully. Patrick's Christian conversion ended slavery, human sacrifice, and most intertribal warfare in Ireland. (He did not banish the snakes: Ireland never had any. Scholars now consider snakes a metaphor for the serpent of paganism. Nor did he invent the Shamrock Trinity. That was an 18th-century fabrication.)
According to Thomas Cahill, author of How the Irish Saved Civilization, Paddy's influence extended far beyond his adopted land. Cahill's book, which could just as well be titled How St. Patrick Saved Civilization, contends that Patrick's conversion of Ireland allowed Western learning to survive the Dark Ages. Ireland pacified and churchified as the rest of Europe crumbled. Patrick's monasteries copied and preserved classical texts. Later, Irish monks returned this knowledge to Europe by establishing monasteries in England, Germany, France, Switzerland, and Italy.
Read Patrick's story, and learn how his legacy was appropriated and made into something entirely different, at Slate. Link