March 17 is still recognized as a holiday in Suffolk County, I believe, and that holiday is Evacuation Day which has nothing (directly) to do with the Irish or St Patrick's Day. The holiday commemorates the departure from Boston of the British Army during the Revolutionary War.
There are three important dates during the New England phase of the Revolution. First is April 19, 1775 which is the battle of Lexington and Concord. Second is June 17, 1775 which is the battle of Bunker Hill. And third is March 17, 1776 which is the day the British evacuated Boston after the Americans fortified the high ground in Dorchester with artillery dragged from Fort Ticonderoga to Boston by Henry Knox during the winter of 1775/1776,
While it's true that the British did leave Boston in mid-March 1776, the historic record is ambiguous about the exact date (more likely it was over several days). But March 17 seemed like a good date, especially when American-Irish politicians gained some influence in the state legislature. Those most likely to oppose a holiday celebrating the patron saint of Ireland would also be most likely to support a Revolutionary War holiday. Hence, we have Evacuation Day on St Patrick's Day.