St. Patrick is the patron saint of Ireland, although he was born in Britain. Many miracles have been attributed to the bishop, including the driving of the snakes from Ireland. His sainthood derives from his conversion of the Irish celtic pagans to Christianity. He used the native shamrock as a symbol of the holy trinity when preaching and brought the Latin alphabet to Ireland. March 17 is the feast day of St. Patrick, of course.
The custom of imbibing alcohol on St. Patrick's Day comes from an old Irish legend. As the story goes, St. Patrick was served a measure of whiskey that was considerably less than full. St. Patrick took this as an opportunity to teach a lesson of generosity to the innkeeper. He told the innkeeper that in his cellar resided a monstrous devil who fed on the dishonesty of the innkeeper. In order to banish the devil, the man must change his ways. When St. Patrick returned to the hostelry some time later, he found the owner generously filling the patrons' glasses to overflowing. He returned to the cellar with the innkeeper and found the devil emaciated from the landlord's generosity, and promptly banished the demon, proclaiming thereafter everyone should have a drop of the "hard stuff" on his feast day.
This custom is known as Pota Phadraig or Patrick's Pot. The custom is known as "drowning the shamrock" because it is customary to float a leaf of the plant in the whiskey before downing the shot.
St. Patrick's Day was first celebrated in America in Boston, Massachusetts in 1737, and is now celebrated nationwide as an opportunity to wear green and consume green libations. The celebration in Ireland is more of a religious matter, whereas in the U.S., it is a festive occasion. The wearing o' the green is a symbol of Ireland's lush green farmlands.
And as alway's thanks for listening!