Saint Patrick was born in Wales in 385 AD. As a child, he identified as Pagan and his friends called him Maewyn. When he was fourteen or so, he was kidnapped by Irish marauders and held as a slave.
In his captivity, he turned to God and wrote:
“The love of God and my fear grew in me more and more, as did the faith, and my soul was rosed, so that, in a single day, I have said as many as a hundred prayers and in the night, nearly the same.” “I prayed in the woods and on the mountain, even before dawn. I felt no hurt from the snow or ice or rain.”
When Patrick was twenty, he escaped from captivity after having a dream from God in which he was told to leave Ireland by going to the coast – there he found some sailors who took him back to Britain. In Britain he had another dream in which the people of Ireland were calling out to him: “We beg you, holy youth, to come and walk among us once more.” With this dream, he believed that God was sending him back to Ireland as a missionary.
Then, he began studying in a monestary to become a priest. For thirty years he traveled through Ireland preaching the Gospel, baptizing thousands, and establishing monasteries all over the country. Patrick died on March 17 in 461 AD at Saul, where he had built his first church.
Patrick incorporated Irish traditions into their practice of Christianity, so he superimposed a sun, a powerful Irish symbol, onto the Christian cross, which became a Celtic cross.
Patrick used the three-leaved clover as a tool to teach people about the Trinity. Each of the clover leaves represent the Father, the Son and the Holy Spirit. Because of this, the shamrock was adopted as a symbol of St Patricks Day.
(Source = http://en.wikipedia.org/wiki/Saint_Patrick