The only pay monthly (I think) gym that I know of in the city centre is the one in the Clarion Hotel in the IFSC. it's probably a bit too far out of your way. It's meant to be nice though,
Markevicz in Pearse Street, Jackie Skelly in Clarendon Street, another gym St. Patricks Cathedral... fitness centres: Pat Henry off Fitzwilliam Square... swimming pool in the Mespil complex on Leeson Street... gym in the Camden Court Hotel next to Bleeding Horse, big gym over spar on camden street. The YMCA on Aungier street.
Few gyms in temple bar - not sure about swimming pools though.
I'm in the Iveagh Gym (between St Patricks & Christchurch). Highly recommend it. There was a signing on fee of about £100 about 4 years ago and I pay €70 a month. Well worth the price if you go 4-5 times a week.
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