St. Patrick the Missionary

Patrick's first encounter with Ireland was at the age of sixteen where he was sent to Ireland and sold into slavery. While he worked in Ireland as a shepherd, he developed a deep faith in god and claims that as he sleep one night, he had a vision of how to escape. After Patrick's escape from about six years of slavery, he went back to Britain, and then to Gaul where he studied under St. Germain for twelve years before returning
again to Britain. In Britain he felt a calling to return to return as a missionary to Ireland; he stayed there for another 30 years converting, baptizing, and setting up monasteries.By the end of the 7th century, Patrick
had become a legendary figure, and the legends have continued to grow since then. There are many legends regarding St. Patrick; the legends state that he used the three-leafed shamrock to explain the concept of Trinity (he refers to it as the combination of  the father, son and the holy spirit). Another legend states that he put the curse of God on venomous snakes in Ireland and drove all the snakes into the sea where they drowned. St. Patrick died on March 17, AD 461 and the day has been commemorated as St. Patrick's Day ever since.

Source:

http://www.theholidayspot.com/patrick/historyofpatrick.htm