Shannon has what it has because of politics.
Orlando can expand in the summer season sure but for most of the year the 1 stop in EWR is enough.
The 500k a year is traffic from NI using DUB to anywhere not just across the pond. They have massive support from the state in various guises. Not least of which are nil apd and border preclearance. While the executive here want their buddies' dinky airport to survive we have no chance of doing much about that.
The closest you will get to another route is a Toronto. I dont see any additional US route as likely.
As for it being cheaper here, I dont check often but I imagine the strong pound is hurting us at the moment