They were doing alot better from Luton, as the route had run for years and was Aer Arann's biggest money earner. Waterford Airport was horrified when the route shifted and their worst fears have now come true.
Stobart invested in Aer Arran as it needed an airline to operate out of Southend and that all Stobart had to do was shift business away from Luton to Southend and the passengers would follow except they didn’t and numbers plummeted. I assume this was because they found Southend Seaside Airport to be too difficult to get to with extended travel times compared to their trips to Luton. It would seem that Waterford passengers don't live close to Southend.
As an aside, historically Essex has been the terminus of WAT flights, not LTN. LTN only ended up with the route because it is where Euroceltic(?) set up and when they went Arann carried on.
I stand to be corrected but that statement is not true. Ryanair introduced the route to Luton which they used to operate into Gatwick. To my knowledge it has never operated from any airport in Essex until it moved to Southend around 18 months ago.