A short distance offshore from Howth is Ireland's Eye, a rocky seabird sanctuary with the ruins of a 6th-century monastery. There's a Martello tower at the northwestern end of the island, where boats from Howth land, while a spectacularly sheer rock face plummets into the sea at the eastern end. It's really only worth exploring if you're interested in birds, although the boat trip out here, with Doyle & Sons, affords some lovely views of Dublin Bay.
As well as the seabirds overhead, you can see young birds on the ground during the nesting season. Seals can also be spotted around the island. Further north of Ireland's Eye is Lambay Island, an important seabird sanctuary that cannot be visited.