I'm wondering--has any Republican ever been mayor of Boston? The influx of Irish immigrants began in the 1840s and by the time the Republican Party was formed, the Irish would have been able to vote. Honey Fitz, JFK's grandfather, was mayor of Boston in 1906, 1907, and 1910-1914. Presumably all of the other mayors in those years were Democrats.
I can't think of one, but my track record of doing this off the top of my head isn't very good :)