Aer Ryan, your comments regarding British (yes, British because Britain is more than England) routes being propped up by mainlanders are unfounded.
The majority of passengers on routes to Britain originate in Northern Ireland, not England. I fly myself, from Northern Ireland to Stansted and Gatwick at least once a month, every month, all year so I actually have personal experience.
The market from Northern Ireland is mostly home grown; inbound and outbound.
Shannon airport won't close down but the other poster is right. The airport is propped by a considerable minority of its passengers utilising TATL (24% in 2015). The airport is much smaller at 1.7 million passengers v 5.1 million at Belfast. Clearly, your comments about the two regions having similar markets are invalid. The two markets are not comparable in size
I don't see Delta coming either (why would they bother if United couldn't make a success of it) but I think you underestimate the Northern Irish market. Something which is easy to do as an outsider (with no experience of Northern Ireland) looking in.