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Reportedly, intra-European award flights are tough to get on EI. And check out the fuel surcharges on DUB-BHX-DUB EI flights: they might be higher than the surcharges on EI's TATL flights.
But yes: adding a stopover of at least 24 hours in DUB on the return will eliminate the higher APD for long-distance travel.
Another option to consider that might save you some $$$ is to book a Reward Flight Saver on BA metal for BHX-LON-DUB. Your travel time will obviously be greater, but the cost savings might make it worthwhile.