1 train out of currently 112 a week stop at 6 stops. 13 a week stop at 3 stops, the rest at 4 stops being relatively major population centres along the route and therefore logical for an Inter-City service.
You need a good foundation to build anything extra. Part of the DART+ extension to Drogheda includes better acceleration/braking than 29000 units and freeing up some capacity that way, likewise with a new Enterprise fleet having better traction capabilities than a 201+DD fleet, and also making Howth-Howth Jctn a shuttle service. My day to day job comes up against a variety of capacity constraints that adding a spur from Clongriffin to the airport will only add to, resulting in a sub standard service.
I want a link to the airport, being a regular user of Dublin Airport, but it has to be done logically. The only way without extra track between Clongriffin and the city centre that you are going to get a decent service to the airport is divert services from other locations. It's not a "circle of negativity", it's being logical about the problem. Otherwise we build a spur, can't adequately use it, and the only winner then is newspaper headliner writers saying another public transport screw up. You don't randomly build a house without checking the land and whether it can support the proposed structure, why should provision of a rail line be any different?
It amuses me that a shuttle is fine to the airport, ie changing trains with suitcases in Clongriffin, but not elsewhere. Either it's OK or not. Why would I get a train from Howth to Howth Jctn, change to a train to Clongriffin, and change again to get to the airport when there's a near direct bus? Same from various other parts of the Dublin area, such as my own on the Maynooth line (when BusConnects reaches us we'll have a direct link to the airport by bus, rather than two interchanges as proposed above. Even today it's just one).