It's just great when you can go to the beach any time during the summer.
These are my tips :
|Rush village beach|
http://www.dublinbus.ie/en/Your-Journey1/Timetables/All-Timetables/33/. Really nice beach and all within walking distance.
If you want to head south from Dublin the best way is to take dart (train) http://www.irishrail.ie/
|Dun Laoghaire pier|
|Dublin coast taken from Bray head|
The further on the coast you go the better Bray, Dalkey and Greystones follow the coast line on the dart. For sure you'll find amazing spots over there. Bray is the biggest town with lots of bars and shops around. Make sure you have some food and drink supplies for Greystones and Dalkey beaches.