In NY city, marketing an unused, unknown, far away airport (Stewart) and an unfamiliar airline (Norwegian) operating to a tertiary market (Belfast) will be a tough task.
The lack of public transport on the NY end of the route will make it an expensive taxi ride to downtown for passengers. UberX is quoting $160 - $200 or up to $300 for an XL.
Norwegian will have to rely even more on Belfast based traffic heading to NY than United. No connections. No US loyalty base. I'd be much more optimistic if this was flying to JFK than Stewart.