While it is true that St Patrick is the patron saint of Ireland, and St. Patricks Day is a quintessentially Irish celebration, it is also the only national holiday I know of that is celebrated by so many, of so many different backgrounds all around the world. It's true that everybody is Irish on St. Patrick's Day.
St Patrick's Day in Moscow
And even in New Orleans
Check out this video for St Paddy's Day with a Mardi - Gras feel, down to the green beads