Most Canadians (like Americans) are visiting the whole of Ireland, not just 1 part of it, and Air Transat are obviously focusing their capacity in Dublin and Shannon in that regard.
That is true of some I'm sure, but many, in fact the most I hear from are visiting family and friends, in both Northern Ireland or Canada. Northern Ireland originating passengers who want to visit family, I'm sure would much rather have the option of a BFS flight rather than Dublin, or as many also do, connect in LHR with Air Canada from BHD.
Again passengers originating in Canada with relatives / friends in NI, where their family have to drive to DUB, rather than the convenience of a BFS flight, or have to connect on the mainland. With family and cultural links between NI and Canada, (as with the ROI and USA), in my opinion would certainly give enough sustainable demand for at least a weekly flight, if not realistically more, as proven by the years of direct flights up until now. If relatives are coming home from Canada every year or two to visit in Northern Ireland, I doubt they'd be trekking around the whole of Ireland each visit.
Now while I have no information on the profitablilty of the BFS zoom flights, I believe thay started with a BFS-YHZ-YYZ, then went to two per week, the second originating in Cardiff, and also added a Vancouver link, also originating in Cardiff. In more recent years the YYZ and YVR operated year round. I would imagine that zoom saw some profit or reason for adding this capacity from Belfast.
Whether BFS got a dedicated flight, or shared with another UK airport (like the SNN DUB or NCL EXT combinations), I'm positive sustainable demand would be there, especially from the VFR traffic.