(Of course, see the 1965 Hart–(((Celler))) Act AKA Immigration and Nationality Act, Hart being Irish, and Teddy Kennedy also in the Senate was said to have been a major help in getting it passed. That's the start of the Great Replacement in the US.)
I would have said that back then, starting pretty much any time prior to the War Between the States when lots started to come over. But I get the impression they eventually did assimilate sometime post-JFK. But could easily be wrong, having left Massachusetts after Route 128 died.
Note, the first major US population group from outside the Hajnal Line. The WASPs could not believe how pathological they were, and they did manage to keep Boston small by not letting it annex the nearby cities and towns a lot of them escaped to. But deportation wasn't in the cards by then.
See the Curly Effect for one well documented example of how bad they could be, but also much more recently the Corrupt Midget William Bulger and his gangster brother, "Whitey." The latter gets us right into Robert Mueller _lethal_ corruption before he transformed the FBI after W put him in charge of it.
And I could well be wrong, see for example the last link, which starts with:
"When you consider that three of the last five Speakers of the Massachusetts House of Representatives resigned from office and were found guilty of criminal charges or copped a plea, you might think you've landed in a lost episode of The Sopranos."
Except two of the three were Irish, the last until 2004. A friend from New York state noted how both corrupt and _incompetent_ at that the Massachusetts political machine was, comparing it on the latter to NY.
https://en.wikipedia.org/wiki/Western_European_marriage_pattern
https://search.brave.com/search?q=curley+effect
https://wbsm.com/corruption-a-way-of-life-in-massachusetts-according-to-report/