Killiney beach is a great place for a walk and easy to get to. To get there by car head south on the N11 until the first roundabout, take the first left and follow the signs. From the centre of Dublin it takes about 30/40 minutes.
There is a free car park at the beach right opposite the dart station. There are no shops nearby, so nowhere to buy ice cream, sandwiches, or water, etc – though sometimes during the summer there is an ice cream van parked on the beach or nearby, but not something you can count on.
The easiest way to get there is by Dart, the local train service. There is a dart station right at the beach. I have no idea re the bus.
The beach is mostly pebbles, though there is a little sandy area just where the tunnel enters the beach area. It is not somewhere for surfing, swimming is fine and it is mostly just swimmers that you will encounter in the water here. It is a great place for a walk, you can walk all the way to Bray and then get the train back along the coast again, either to the car park to collect your car or right back into Dublin again.
I must have walked up and down Killiney beach at least one hundred times, my other half is from a land locked country and every weekend she loves to see the sea. You can also view up and see the big houses above the beach that have some of the best views in Ireland. One of these house is the home of Bono of U2 fame.
This area is outside of the main Dublin Bay area and hence the water looks much cleaner. Sometime you can see seals, however they keep mostly to the islands just off Killney. If you really want to see the seals the best way is by going on the Sea Safari – a motor boat that gives tours across the Bay of Dublin.