A couple of articles mention that Norwegian will open four transatlantic bases; two in Europe (EDI and RIX) and two in the US (PVD and SWF). The logistics of a Belfast base wouldn't make any sense. Basing the aircraft in the US would allow them to fly to BFS, ORK, SNN and would give them room to expand to other European markets. Their transatlantic Belfast operation probably won't expand beyond two destinations, so why would they base the aircraft on this side of the Atlantic if their operation is much bigger on the other side.
Norwegian set to open up to four 737MAX bases for 2017 transatlantic growth :: Routesonline