Think in all honesty there is good North America demand from NI. Otherwise, why would Dublin be advertising so much about US flights in Northern media??
’Norwegian Air’ flying irregularly over the pond from Belfast to somewhere in upstate New York was always a hard sell, especially when there are so many known ways to get there. Rumoured more than Belfast and Edinburgh are a challenge on the transatlantic network, even with low fuel costs. Likely the big problem for Belfast, maybe even Shannon and Cork, is getting enough of the US visitor market, when everything is sold to Ireland citing Dublin as the arrival point.
Maybe the easiest way to go is get rid of air tax and grow a bigger range of European routes?