First, from Wikipedia:
Saint Patrick (Latin: Patricius; Proto-Irish: *Qatrikias; Modern Irish: Pádraig [ˈpˠaːd̪ˠɾˠəɟ]; Welsh: Padrig) was a 5th-centuryRomano-British Christian missionary and bishop in Ireland. Known as the “Apostle of Ireland”, he is the primary patron saint of Ireland, along with Saints Brigit and Columba.
The dates of Patrick’s life cannot be fixed with certainty but, on a widespread interpretation, he was active as a missionary in Ireland during the second half of the fifth century. He is generally credited with being the first bishop of Armagh, Primate of Ireland.
According to the Confessio of Patrick, when he was about 16, he was captured by Irish pirates from his home in Britain, and taken as a slave to Ireland, where he lived for six years before escaping and returning to his family. After becoming a cleric, he returned to northern and western Ireland. In later life, he served as an ordained bishop, but little is known about the places where he worked. By the seventh century, he had already come to be revered as the patron saint of Ireland.
Saint Patrick’s Day is observed on 17 March, which is said to be the date of his death. It is celebrated inside and outside Ireland as a religious and cultural holiday. In the dioceses of Ireland, it is both a solemnity and a holy day of obligation; it is also a celebration of Ireland itself.
The Shamrock – The shamrock was used as a metaphor to teach pagans the concept of the Trinity, how the Father, Son and Holy Ghost are separate, like the three parts of the shamrock leaf, but one, in that they all form a single leaf or entity.
The Snakes – The story of St. Patrick driving the snakes out of Ireland is an allegory. Like a metaphor, it uses one thing to represent another, but instead of a single object, it is a story. In the case of snakes, St. Patrick actually was credited with driving out paganism, the form of the serpent or snake a metaphor for the Devil and evil, as in the Garden of Eden. By bringing Catholic Christian beliefs to Ireland and spreading them quickly, he “drove the snakes out of Ireland.”
So there you have it.
Erin go bragh!