Mart901,
If Norwegian entered Belfast, they will not pull many away from Dublin. With daily service to New York by 737 (lets assume a 189 seats), at 100% fill they will marginally improve over the numbers achieved by the daily 757 of UA.
More importantly:
Flights to the USA from Dublin are carrying a significant amount of passengers from points in the UK and Europe, not just EI but OALs too e.g. Finnair, Turkish, Lufthansa, Swiss, Air France. Star, Sky Team and One World are all well represented in Dublin.
UA have said that 60% of their traffic goes on from EWR and EI are seeing a similar proportion of traffic not flying P2P, i.e. flying on from US gateways.
More importantly a greater share of tickets for travel to/ from or through Dublin are purchased in the US, not just for EI but the US carriers to.
Though small the numbers that Icelandair pick up at BHD (these will be destination US/CA), these will eat into what a dedicated service ex BFS would deliver, this should be added to the overall mix.
So as much as I like to see Belfast Airports prosper, there is no threat posed here to DUB, and with a brand like Norwegian who to date have no onward feed in the US and little brand awareness(though I don't underestimate this will change), I am not convinced that they will see that great of uplift in numbers when compared to UA, as like through Dublin, a lot of people who flew UA were going to other points in the US and not just New York. Data I have seen recently for the whole Belfast market shows that other points can be just as popular as NYC, like LAS, MCO, SFO and BOS, and a lot of this is low yield Leisure business.
IMHO, WOW would be a way better fit for BFS right now. Feeding passengers through KEF offering a range of destinations not just in the US but also in Canada, a destination that has been popular in the NI market in the past.