I asked a similar question recently. There is actually a gym at Griffeen Park (the entrance is just beside then entrance to the Ash Park estate). I don't think they have a web site, but they do lots of classes and have a gym as well.
I'm in Westmanstown. Its only 535 euro and its got really nice gym, 25 m swimming pool and tennis courts and classes. I think its cheap coz the guards subsidise it. Just renewed for another year. They also do a couple deal for 965!