There is some confusion as to when Patrick was born, but it is believed to be between 373 and 385 A.D. in Scotland. He was the son of a minor Roman official (the Romans didn't leave the British Isles until 410 A.D.) and was named Maewyn Succat.
When Maewyn was 16 years old, he was captured by marauders from Ireland and sold into slavery to a chieftain named Milchu from Dalaradia. For 6 years he tended sheep and acquired a perfect knowledge of the celtic tongue (this would come in handy for him later).
While tending sheep, Maewyn grew closer to God and prayed several times daily. After being a slave for 6 years, he escaped, traveled 200 miles to the nearest seaport, found a ship about ready to sail, and headed for Britain. But, even though he was among his friends again, he felt that he was being called to the ministry and so he traveled to Gaul where he studied in the monastery on the island of Lerins under St. Germain, the bishop of Auxerre for twelve years.
While studying to become a cleric, Maewyn changed his name to Patricus (Patrick) and realized that his real calling was to go to Ireland and convert the pagans to Christianity. I would guess they didn't feel he was ready for this task, because they sent a man named St. Palladius instead. After 2 years, however, Palladius went to Scotland and Patrick got his wish to go to Ireland. He was appointed the second Bishop.
When he arrived in Ireland, he set right to work converting people. Since Patrick had a winning way about him, he was quite successful. This didn't please the Druids, who had been the main religious force. They challenged him many times and because of his faith, he was able to meet the challenges and win. He was also arrested several times, but managed to escape each time.
Patrick traveled all over Ireland for 30 or so years, establishing schools and churches to aid him in his converting the Irish to Christianity. He died on March 17, 493 A.D. and this date has been commemorated ever since. Many folklore tales surround St. Patrick and who knows what's real and what isn't?? But what is true, is that Maewyn Succat, Patricus, St. Patrick was a remarkable man who accomplished a remarkable feat is converting an entire nation of people to Christianity.
One of the biggest legends surrounding St. Patrick is his driving all the snakes out of Ireland. In actual fact, he drove them into the sea. Snakes were big symbols in druidism. Without the snakes, a lot of their power was lost.
"One traditional icon of the day is the shamrock. And this stems from a more bona fide Irish tale that tells how Patrick used the three-leafed shamrock to explain the Trinity. He used it in his sermons to represent how the Father, the Son, and the Holy Spirit could all exist as separate elements of the same entity. His followers adopted the custom of wearing a shamrock on his feast day."*
"Celebrations outside of those in the church began in Boston with the Charitable Irish Society, founded in 1737. The Friendly sons of St. Patrick, founded in Philadephia in 1780, observed the day and four years later in New York the Friendly Sons of St. Patrick of that city took note of the anniversary. The New York society, curiously enough, was organized by Irish Catholics and Presbyterians and its first president was a Presbyterian."**
*(Quote taken from St. Patrick's Day - Customs and History)
**(Quote taken from McLeansboro.com: St. Patrick's Day History & Myths)