This discussion shows up the dysfunctional governance of public transport investment decisions in Ireland. Major projects such as Motorways, rail electrification and new routes, main airports, all involve big external impacts (e.g. a new motorway will impact on some main rail lines, an expanded airport will impact on the demand for different modes of surface transport). As such they should not be regarded as "ordinary" investments where the only relevant measure is the financial impact on the investor.
This is why we have bodies such as the NTA, and a Department of Transport. They exist to regulate transport providers and co-ordinate big investment decisions. If a major airport acts like a car-park provider and retailer with some runways attached (OK I know this exaggerates a bit) this is clear evidence of regulatory failure. Thus a rail link to Dublin Airport should be an NTA/Government decision and the criterion should be based on costs and benefits to society as a whole, and not just to DAA (or Irish Rail for that matter).
If we have a minister of transport whose main concerns (or even obsessions) seem to be the appropriate penalties for driver misbehavour or for the procedure used to appoint judges, then it is almost inevitable that major questions of governance and regulation in the transport sector will be neglected, with huge long-term consequences.