The area: Ballawley Park is a pleasant public park in the Dundrum/Sandyford area, with two playgrounds for children and a series of outdoor gym equipment for adults. There is no shortage of trees and green areas, so take a stroll and find the other caches in the park while you're here.
The cache location: Situated off the main paths up a grassy corridor that starts to look slightly more like a back garden than part of the park, you have reached the hidden corner of Ballawley Park. You probably won't find many muggles here. 
There are some houses nearby, but not so close that searching will look suspicious. Aside from that, just follow the GPS... Please put the cache back properly to avoid obvious visibility, and don't post spoilers about the container! A tweezers is advisable to retrieve the logbook and avoid damage to the container.
Parking: There is a small carpark on the Sandyford Road opposite Balally Hill, which brings you through the main entrance on the opposite side of the park. If you want to minimise the walking you have to do, park at the top of Wesley Heights instead (note that this is part of a housing estate, so if you're not familiar with the area then parking on the Sandyford Road might be easier).
Credit to gaffer.gamgee for the container!
Congraulations to tmsr for a very quick FTF!