Many say the true Ireland is not to be found in the city and if you are here during the summer season you would be well advised to spend some time in the wider Dublin region.
You'd be surprised how many nice sandy beaches you can find out there. During winter a spa could be a good option, though most Spas with saunas, swimming pools and gyms, are part of hotels and only for guests. The case is similar for most gyms which require a membership. One of the exceptions is the university gym.