Hey OP,
I've been checking this out myself over the past few days. I also contacted Jackie Skelly's and was quoted almost €800 for membership. Was considering the Fitness Dock on Lower Camden Street as it's convenient to work, but the 2 for 1 offer, which wouldn't apply to me (I'm on my own) is only for a six month membership. Peak annual membership works out around €575.
Found some information on the Markievicz Leisure Centre located on Townsend Street. It seems to me to be the best around at the moment, though I have yet to check them out. Intending to do that next week. Haven't been able to discover how much the annual membership is but they offer a pay-as-you-go service, which would allow you to try the facilities before becoming a fully-fledged member.