Hmm. It would bring more competition to Belfast-Heathrow, but it would still be another route monopoly - EI to BFS versus BD to BHD. Given the amount of high revenue and government traffic to BHD, I know which route I'd rather monopolise!
The other issue is slots at LHR. I presume EI would have to reduce its DUB-LHR frequency to be able to launch LHR-BFS. This would reduce the advantage EI currently has over BD on that route.