Thanks to Kevin F. for this recommendation and providing the information below.
Probably the nearest swimming spot to Dublin city. Good changing areas and even coffee shop. Lifeguards on duty in summer. Steps down to water so you don’t get sand everywhere!
You need to be within a couple of hours of high tide or there will be not be enough water. Amazingly, not nearly as busy as 40 ft. or Seapoint for swimmers. Crammed with walkers though. (source: Kevin F.)
Cross the very charming wooden Bull Bridge at Clontart to get over to the North Bull Wall. While to beach is on the left hand side of the wall as you walk towards Réalt na Mara (the statue of Our Lady, Star of the Sea), to the right of the wall, you will see a series of shelters where you can get changed (pay attention to the Gents/Ladies labels!).