Sure the holiday is mostly popular because of the Irish in America.
St. Patrick's Day was a somewhat minor holiday in Ireland until Irish-Americans (likely wanting a taste of home) helped make it the huge celebration it is today.
And, though Guinness is my favorite beer and I celebrate today with the best of them, it should be noted by everyone (Irish or not) that St. Patrick was a rather interesting fellow. Certainly one worth reading about.
And for nearly all you ever could know about St. Patrick - here's a great site.