EI-BUD
There is no evidence that "SEN-DUB does not work" because when it was a 3 x daily Aer Lingus Regional (Aer Arann/Stobart Air) route Stobart stated that they hoped to add a 4th rotation - hardly indicating the route was struggling.
Indeed, as you say, it looks bad for them to drop SEN-DUB in terms of attracting other non-sunshine routes to SEN. So even if profitability was "marginal" it should still have made sense to continue it to showcase SEN's potential in that market (particularly as it offered attractive connections to US cities). Also traffic on the route was still growing prior to its handover to Flybe so its full potential hadn't been exhausted at that time. Once Flybe dumped it I find it hard to believe that Stobart couldn't find the capacity to continue it.