Gym's are not really my thing but my wife's attends the gym (pool) at the Glenroyal Hotel in Maynooth and there is also the Leixlip Amenities Centre which has five aside pitches etc.
I believe there is also a gym found in Captains Hill called Motions Gym but never been so can't comment.
There are number of others a bit further a feild a quick google should finds these
Motions gym has been closed for a long time now, theres Crunch Fitness at Westmanstown which I am a member of, wouldn't rate leixlip amenities gym too highly. Leixlip United are a good foolball team in the LSL, think they train and play matches at the amenities centre
Leixlip United train in the Leixlip Amenities Centre (beside the tennis courts) starting at 7:15 every Tuesday and Thursday nights. It is a high enough standard of football in the Leinster Senior League if you are interested in joining. I think registerations and transfers must be done by end of Sept.