The most famous legend of St. Patrick says that after forty days of fasting on the mountain now called Croagh Patrick, he came down and was menaced by a gathering of snakes. Patrick was so angered that he took up his staff and drove the snakes into the sea where they drowned. But, it's just a legend, probably symbolic of Patrick's efforts to convert the Irish from paganism to Christianity.
The real Patrick was born around 397 A.D. in England or Scotland (there seems to be some debate as to his specific birthplace) to high ranking Roman parents and was kidnapped and sold into slavery by Irish pirates around age 15 or 16. He would remain a slave to Milchu, an Irish chieftain, for six years, and during that time learned the Celtic language and Druidic traditions. Patrick escaped in his early 20s, supposedly after a vision from God encouraged him to head for the coast where he was picked up by sailors who took him back to Britain. Years later, another vision led him to become a priest and return to Ireland as a missionary. Patrick studied in France with the guidance of St. Germain, the bishop of Auxerre who would ordain him as a priest. At age 43, Patrick became the new bishop of Ireland and set out on his mission in 433.
Down Cathedral. Said to be the burial place of St. Patrick
Patrick met with some stiff resistance from the druids, but overcame their magic and assassination attempts with some possible divine intervention. Patrick's victories against his challengers either impressed and/or terrified Laoghaire (the Irish high-king) so much that he granted missionaries permission to preach in Ireland, as well as the promise of safe passage. By 444, Patrick had already converted scores of Irish people, but Laoghaire wasn't one of them. The shamrock is said to have served as the symbol Patrick used to teach converts the concept of the Holy Trinity. It was effective, probably in large part because the shamrock already symbolized the important stages of life in ancient Celtic tradition. Patrick continued his work for the rest of his life until his death on March 17, 461. Ironically, he has never been officially canonized by the Catholic church.
If you're in Dallas for the Greenville Ave parade, don't be these guys
St. Patrick's Day was established as a Christian feast day in the seventeenth century and was celebrated in the American colonies in the eighteenth. The Irish Potato Famine of 1845 caused a wave of immigration to the U.S. and the parades grew with the diaspora. St. Patrick's Day has become more secular and commercialized over the years, but that hasn't rained on the parade (I'm sure Patrick will forgive the pun) of Irish and non-Irish all over the world.