Alright, St. Patrick is the Patron Saint of Ireland. By the time of his death on March 17, 461, he had established schools, churches, and monasteries. Legends grew around him, like driving snakes out of Ireland.
Interestingly, the first St Patrick's Day parade was not held in Ireland at all, but in Boston in the United States in 1737. So while the yearly commemoration parade in Dublin, Ireland is huge, theatrical and colorfully green, the celebration is something that was actually brought into Ireland. In fact many St. Patrick's Day practices like coloring the beer green was brought into Ireland instead of stemming from it. Shamrock however is the national plant of Ireland.
But then again, St. Patrick himself was not even from Ireland. He was born in either Wales or Scotland where he was taken as a slave by Irish raiders and brought into Ireland. He was later able to escape back to his family in Roman Britain, only to go back with Catholicism in tow. He brought luck too and as the legend goes, left Ireland snakeless. Until this morning.
Wearing his symbol, the Shamrock leaf, on St. Patrick's Day is a celebration of Irish culture and is said to call on St. Patrick's blessing. It has also been said that this is made more potent by mixing lucky four-leaf clovers in. Like on your tie.