Irish Rail's reports favour 2 tracks on western side. 3 tracks possible within existing boundary. 4 tracks would require land acquisition. Every station and bridge would need to be rebuilt
The problem is Rahney to North of Howth Junction, there was no way to reasonably fit 4 tracks, only 3. Now the proposals suggested grade separation at Howth Junction. The Airport spur would be grade separated and merge just north of Clongriffen which is built with 4 platform already
The scale of disruption required would be immense and that is a serious concern
Should be noted the DART, 29000, ICR and Mk4 fleets are all designed to cat B fire standards and can operate in long tunnels. All you have to worry about is ventilation.
Cost wise to get 4 tracks in to Clongriffen would be 600+ million, add 400 million to get to the Airport and some extra trains say 100 million you are already 1.1 billion down, add in 4 tracks to Donabate could be another 400 million. But your airport spur is 8km the wrong way so a very slow route if you want to do Ballbriggan Airport Dublin its out as is Dublin Belfast via Airport as it adds 16km to the journey
Our tunnel option plus the loop back towards Rush and Lusk would be ballpark 2 billion, but its direct and avoids the need to grade separate Howth Junction, dig up Killester Donabate, avoids Malahide Esturary and provides a new service from north of the Airport. Best of all in distance terms its 2-3km extra at most but without the curves or traffic of the existing route so you can fly along at full speed. All Ballbriggan/Drogheda/Dundalk/Belfast would divert via tunnel. DART from Rush and Lusk on existing route. P&R and interchange at Rush and Lusk
Ignoring the complexities in isolation the spur doesn't work you need 4 track and interconnector and if you are going to go that far you might as well get the full package
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