A long stretch of sand on Bull Island, a man-made island in Dublin Bay created by the build-up of sediment behind the North Bull Wall. The beach has Blue Flag status and is popular with walkers, kite surfers, and families on sunny days, though the tide goes out a long way so swimming can be hit or miss. At the northern tip you'll find a nature reserve that's home to wintering waders, Brent geese, and a small seal colony.
Take the 130 bus and ask the driver to let you off near the old wooden bridge. The island itself is also a UNESCO Biosphere Reserve, worth a walk even on a grey day.