From the US, Dublin yes - Belfast no. Flights to Dublin (Shannon too) can be ridiculously cheap, particularly in winter. I flew nonstop JFK-DUB on Aer Lingus in February for about $430 r/t. There's a fair bit of competition on that route, and EI is sort of positioned these days as a low-cost carrier, so you can often find very good deals if you're flexible. US also services those airports and has very economical international fares, so that helps as well.

Belfast, on the other hand, only has one flight to the US - a United nonstop to EWR. It's usually far more expensive than the other airports in Ireland. I'm going the week after Thanksgiving and was lucky to find a seat for <$700 r/t on that flight.

Even from London, I think it's much cheaper to fly to DUB than either Belfast airport. Frankly, I don't think there's a whole lot of tourist demand for Northern Ireland. You go there either because work sends you or because you're going to see family, and the demand for that type of market is relatively inelastic.