Great South Wall - This is my all-time favorite, mostly because it's so close to my house and pairs neatly with a picturesque half-hour bike ride there through Sandymount and the Irishtown Nature Reserve. The Great South Wall was built in the 18th century along the Liffey to help prevent silting from getting in the way of shipping. It's about an hour walk along the seawall from the parking lot out to the lighthouse and back again, with stellar views of the whole of Dublin Bay from Howth to Dalkey and the Dublin Mountains in the background.
Howth Cliff Walk - The best thing about the Howth cliff walk is how it's so easy to feel like you're way out in the country but actually be only a short DART ride away from home. On the far side of the peninsula there's just cliffs and sea, but the nearer side offers a charming lighthouse and some pretty views of Dublin. The full walk is a 3-hour loop all around the almost-island of Howth but there are shorter loops for the less committed. There's plenty of delicious food in Howth village for brunch/dinner/snacks on either end. I love this walk best in late summer/early fall when you can pick juicy blackberries off the bushes as you go.
Bray Head - The short but steep hike up to the top of Bray Head can be a bit punishing for the less athletic among us, but the views are worth it. There's a little-used path down on the south side that takes you through some more hills (especially pretty in the spring with the gorse in bloom) and then to some less official trails down to the cliff walk and thence to Greystones to celebrate your acheivement with tea and cake. Those unenthused about elevation can just take the cliffside path from Bray to Greystones (or vice versa), which is nice too and much less strenuous.
Malahide Castle - This one requires some monetary investment to get the most out of the experience, but it's well worth it. Buying a ticket to the castle (interesting in its own right) also gets you in to the lovely gardens and grounds, home to an incredible array of plants from all over the world. There's a magnificent 300-year old cedar, gorgeous flowers everywhere, and even a pair of peacocks to add to the glamour of the scenery. And if you get bored with parkland and gardens you can always wander down to the village and the coast for some pretty seaside views.