I would see the frequency as needing to be at least every 15 minutes. I agree it would need to start a lot earlier than the present DART system - probably early and late enough to allow people to take it to work at the airport.
But my question would be why bother build it at all if it is no better than the present bus arrangement. If it's so we can keep up with those fancy europeans with a shiny impressive railway to the airport, maybe we should think about why. By better I mean, faster, more integrated or more comfortable. There is some argument towards having it to give arriving tourists a good impression of the country but Irish Rail would want to seriously up their game for that to work.
There is a possible argument for integration but that is dubious at best with the present rail network. As it stands a huge number of Bus Éireann buses already go via the airport and an express bus can get you to Connolly or Busaras in 20 minutes through the tunnel. If DART underground ever gets built, then the possibility of Airport - Connolly - Heuston suddenly starts making a lot more sense but if there is no plan for this to happen, the airport spur is a waste of money.
The other problem is that it will be bunging another 4 trains an hour and all the attendant crossing of the existing line onto a corridor where Irish Rail already have massive difficulties maintaining a timetable. While the city centre resignalling project will help with that, there is so much rubbish floating about at trains for Galway, Cork and Limerick running straight to Connolly, that they'll need to be able to put 30 trains an hour over the loop-line bridge.
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