Vanishingly unlikely. Boston is full of Irish people, so for an Irish terrorist group to set off an explosion where they're more likely to injure Irish people than elsewhere makes no sense. If they were to set off a bomb in the US, which has never happened, the obvious target would be a courthouse or US attorney's office.
I'm Irish and intimately familiar with the politics of terrorism, FWIW.
Well, then you know Irish terrorist groups placed quite a few bombs in places like Manchester, i.e. a city with closer links to Irish people than almost everywhere else in England.
To be fair, these attempts backfired spectacularly, losing them a lot of support in these areas very quickly.
I'm Irish and intimately familiar with the politics of terrorism, FWIW.