No wish to be a party-pooper but I have yet to see anything from any airline saying Southend-Carlisle will definitely go ahead. Cannot see anything like a flight schedule, aircraft type, published fares or tickets on sale.
My guess would be Stobart Air/Flybe as the operator as they would suit the domestic routes from CAX. As for DUB, they offered onward EI transatlantic connections from SEN before, so surely they could do the same from CAX. I can't see EIR wanting to operate UK domestic routes.
As part of the planning permission for the development of CAX, Stobart committed to operating an ATR-42 to SEN and DUB (Belfast is a relatively new development). These flights have also been mentioned locally in every stage of the airport re-development (though I'd agree with DJ6 commitment to a start date has been "flexible").