I live out in Skerries, Most weekends there is motorhomes parked up along the south strand, just south of the public toilets.
Google maps won't let me place a pin and share it for some reason, but if you see the map below the area marked green is where they park.
There was a water tap at the public toilets, but its been soon long since I walked past that I don't know if its still there.
If you want to know anything let me know, if I remember this evening I will drive past on my way home from work and see if the tap is still there.