Some people believe the American version of St. Patrick's Day is a transplant, but our St. Patrick's Day has very little to do with Ireland. Until the mid-1970s, pubs in Ireland were closed on March 17: It was a religious holiday, the feast day of the nation's patron saint. You went to Mass, had dinner with the family, and if you knew a pub where the back door might be unlocked, you might sneak out for a pint. Only since the mid-1990s has Ireland, and Dublin in particular, promoted public revelry as a way to convince tourists to come to Ireland in early March and get rained on.
As a public boozefest, St. Pat's is originally an American phenomenon, a product of the ethnic conflict that has defined urban America from the early 19th century onward. St. Pat's parades did not begin as citywide celebrations of all things green; they began as defiant statements of ethnic and religious solidarity in the face of discrimination. Irish Catholic immigrants of the 1830s and '40s were America's first urban underclass; they were often called "white niggers" (while blacks were sometimes called "smoked Irish"), and St. Patrick's Day parades were a defiant slap at drink-at-home-if-at-all Protestant propriety. St. Patrick's Day parades were a provocation, and they often devolved into anti-immigrant riots; stones were thrown at St. Pat's processions, and the occasional Catholic church was burnt to the ground.
How things have changed. The root of the leprechaun-and-shamrockification of St. Patrick's Day and its parades parallels the assimilation of Irish-Catholic Americans into the dominant culture. As the Micks and Paddies became far less threatening to the WASPs than the Stashes and Luigis immigrating from Eastern and Southern Europe, the Irish moved up the socioeconomic ladder, and St. Patrick's Day was watered down into Shamrock Shakes at McDonald's. The Irish learned to drink beer from their German and Scandinavian neighbors (whiskey was the Irish immigrants' drink of choice), someone discovered green dye, and the political power of the urban Irish made St. Patrick's Day a civic celebration and political event at which to troll for votes--and drink in public.
Public drinking became acceptable on St. Pat's in places like New York, San Francisco, Boston, and Chicago because of a tacit agreement between the drunken revelers (we'll keep property damage to a minimum), the police (we'll be joinin' ye as soon as our shift's over), and politicians who didn't want to be bothered with jails full of hung-over voters on March 18. These historical changes--along with vigorous marketing by breweries and liquor distributors--has transformed St. Pat's from a political brawl into an annual drunken sing-along.