Dave,
Take the left turn (if travelling from Dublin) at Jack Whites and follow the road all the way down to the T junction, turn right, follow that road past Brittas Bay.
You eventually pass the Europen Golf Club on your left, keep going.
When you come to a small bridge (river underneath), you will see a gate on your left - thats the entrance to North beach Ennerielly, the south beach is a small bit further along, you will see an opening on your left where you can park, and the beach is right in front of you.
Image below, zoom in to see the river, the red marker is the North Beach.
http://mapcarta.com/18955914Hope that helps.