It Entirely depends on where you are interested in flying from, and if you have the money to maintain and keep, as well as buy and fly a plane.
Weston is anywhere from ?2,500 a year outside, or upwards of ?10,000 indoors a year. But it is on a bus route, and 45 minutes from Dublin City Centre... just outside Lucan
You could also look at Trim, Trevet or Newcastle, although all are a fair distance from Dublin. If you live in Dalkey, Newcastle, depending on how you deal with the owner of the private field would probably be the closest, still a good 40 minute drive though.