That history is - surprise, surprise - closely related to the region's race and class relations. Boston has always been the flashpoint for such issues, more so than New York City, in part because of its smaller size and more pointed and concentrated relationship with the slave trade. The two great waves of Irish immigration meant tensions among the working and lower-middle classes, which had previously been largely black - tensions which were gladly encouraged by New England's wealthy. Incorporating Irish into the police force was a direct (and highly successful) attempt to foment interracial clashes; this preempted the forming of interracial bonds within those lower classes, which would have allowed them to demand more rights and a better quality of life from the elites. This is the same thing that happened with labor unions; Irish and black fraternization is bad for business, so court the Irish and foment bigotry against their black neighbors (to the point where they don't even want to
be neighbors anymore).
The way that this dynamic extends into the busing crisis and the infamous racism of Boston sports fans should be obvious.