Legend says St Patrick was born in Britannia in what is now Scotland and was then under Roman rule. He is believed to have been of noble birth, kidnapped a held by the Irish for 6 years. After he escaped from slavery, he returned to Ireland to preach and to convert the Irish to Christianity. Here is the story of the shamrock from Wikipedia:

Patrick described the trinity to his followers by referring to the clover or shamrock. This was a sacred plant to the Celts, honoring the triple Goddess, so using it as his analogy helped to create a bridge from the ancient to the new religion and contributed to his many “conversions.”  This is the origin of the shamrock emblem we still associate with him. The leprechaun, also associated with St. Patrick’s Day, is an obvious reference to the elementals of Pagan Ireland.

Corned Beef tips from the USDA

Check out St Patrick’s Day.com

St Patrick from Scotland Online