Here is some interesting information that came to my attention today.
The four-leaf clover is often confused with the shamrock. While the four-leaf clover is a symbol of good luck, the three-leafed shamrock is mainly an Irish Christian symbol of the Holy Trinity and has a different significance.
The shamrock is a symbol of Ireland. It is a three-leafed old white clover. The shamrock has been registered as a trademark by the Government of Ireland.
The shamrock is a traditional icon of St. Patrick's Day because it tells how Patrick used the three-leafed shamrock to explain how the Trinity of the Father, Son, and Holy Spirit could exist as separate parts of the same being. His followers took to wearing a shamrock in celebration.
The St. Patrick's Day custom came to America in 1737. That was the first year St. Patrick's Day was publicly celebrated in Boston. Today, St. Patrick's Day is celebrated with large parades, the wearing of the green, and drinking beer.