Here in Toronto we're about to kick off
a service between Pearson and Union Station with two stops in between. It was deemed necessary to build two dedicated tracks to get this done. Supposed to operate 19.5 hours a day, 4 times an hour per direction (initially with DMUs)
Instead of extending DART wires over a spur and spreading the DART fleet more thinly, how about insisting that a 3rd Northern Line track be the priority, with airport service being then operated by refitted 2700s with additional luggage space on a semi-express basis (DUB-Howth Jct-Connolly-Pearse-GCD-turnback). A third track would offer the opportunity to increase total capacity rather than crudely carving out some of the existing, creating resistance to/resentment against the airport service.