I guess you will be right, although I wonder if it will be BCN-ORK-BOS-ORK-BCN, at least until New York comes on-line, they haven't said anything about 'basing' an aircraft at Cork, I don't think?
European crew require a visa to enter the US. The last thing Norwegian will want to do is arrange visas for a small number of BCN based crew to operate this route
Norwegian currently employ over 350 US based cabin crew. The crew are based at FLL and JFK and currently operate all Norwegians flights to and from the US. Was talking to one of there JFK crew last month and she said that crew based in JFK are going to be trained on the 737 in the very near future. So I think it is more likely they will operate the BOS and BCN based crew will operate the BCN using two different aircraft.