If I'm not mistaken the FOI (Freedom of Information) Act applies to government documents - not private documents. The Catholic Church would be exempt.
This could be a case of the right hand not knowing what the left hand is doing. Finding out where all the records from the now non-extant church (where your grandmother was married) is the challenge.
You are asking your question on the Ireland Forum. If you go to a Massachusetts Forum or even a Boston Forum and ask about the specific church records that you are interested in you might get a response from someone who has already traveled this road.