There is a gym in Citywest, its quie an old gym so i wouldn't recommend it but there is also a lovely one in tallaght, on the green hills road, its called westpark. very nice but its quite pricy about €650 a year, they are all around this price.
I think the gym is Citywest is lovely. Its not modern but is in beautiful grounds and has very good 'pampering' facilities (sauna/steam room/lovely changing rooms) so it doesnt feel clinical like the newer gyms. There is also a nice pool and membership is affordable enough. I think a joint membership is €50 each per month, not sure about individual membership.