Pierre
If you want sandy beaches then certainly the north side of Dublin is the place to go. South side beaches tend to be gravelly and sand tends not to be the fine variety.
But if you want fantastic coastal scenery then the south side is the place to go, with the exhilarating cliff walk in Howth the honourable north side exception.
The view of Killiney Bay, the city of Dublin and the Dublin and Wicklow mountains from the top featured in 'Once' and 'My Left Foot' if you want to skip the walk up the paved path to the summit. The cliff walk from Bray to Greystones (or Greystones to Bray obv) offers a good stretch of the legs and the experience of just you and your thoughts and the sound of the sea. All of this is free and within easy reach thanks to the Dart.
Again accessible by Dart, you can rent a sea kayak at Bulloch Harbour, or take a boat tour to Dalkey Island and see the ancient Christian ruins of St Begnet's church, a cairn burial site, two holy wells, and the Martello Tower that was constructed using stones from an old druidic stone circle that once graced the ancient Bulloch settlement. If you're self catering maybe you would like to buy a fresh lobster or some crabs or mackerel early one morning from the working fishing boats that use the harbour.
Or visit Coliemore Harbour, where you can also embark on a tour of Dalkey Island or paddle around in a kayak, or just sit and watch for dolphins and seals. Jump in and ply the bracing waters if you time the tide right.
You can also swim at Sandycove further north along the coast south of Dun Laoghaire, or at the Forty Foot if you prefer not to wade in and you feel like thumbing your nose at the patriarchy.