I don't think full four tracking will happen due to space constraints, but it certainly could happen after Clontarf Road and the far side of Raheny - that would allow for overtaking, along with the installation of an up passing loop at Clongriffin.
I'd agree - the airport spur would need the above to happen first.
I don't either, well without underground sections.
What could happen and at reasonable cost is a third track between Clontarf and Howth Junction (as much of the route as possible) and operated in a single direction at peak hours and however IE like outside of it. The signalling would not be an issue and it would deliver a remarkable difference in service to people outside of the DART line and allow 10 minute DART service while cutting times for diesel commuter service. Could even cut costs by not electrifying the third track immediately.
I don't travel the route a lot but allowing Commuter and Enterprise get up to speeds for 70-80 after Clontarf Rd would surly cut at least 5 minutes if not more at peak hours....all they get is Danger signals today!
Lots of solutions but I'm not sure who is worse IE or the NTA for complete lack of planning on this route.