Portmarnock is a coastal town on Dublin's northeast coast, just south of Malahide. There isn't a lot to do there but it has a lovely beach. I recommend a trip if the weather is particularly hot and sunny.
Great beach to relax. It is pretty long and has clear waters, grass and it is very clean as well. It can be crowded in warm days, but is big enough to be enjoyed. You can get there with bus 32, which you can take from Fairview Park.