If the authorities in the Republic were serious about Dublin Airport, they would build a rail spur from the Belfast to Dublin line from north of Malahide to Dublin Airport and onwards through the Phoenix Park tunnel to Heuston station connecting up Cork, Waterford, Limerick, Galway, Kerry and all the stations along those rail lines to Dublin airport but it ain`t going to happen.
That would be the plan but it cannot happen until DART underground is build, the city centre doesn't have the capacity to allow this and the Phoenix Park tunnel isn't fit for purpose as Connolly station couldn't handle such traffic, currently its more less at capacity but works are being carried out to increase it to 20 trains per hour but it won't be enough to allow full usage to airport from regional cities. Dart Underground is the only way for it to work.
The whole project is being reviewed in 2015 when the next investment plan is announced and this project could have 2.2 billion made available for it. It would cost around 4 billion to fully complete. All major road projects have being completed so this is most likely to be granted. as many people want to and it will have the most benefit to transport in Dublin.