A squat granite Martello tower on the rocky promontory at Sandycove, about 1.5 km along the coast from the main harbour. It was built in 1804 as part of a chain of defences against a Napoleonic invasion that never came. James Joyce stayed here briefly in 1904 and used it to open Ulysses — the tower is now a small Joyce museum, with first editions, letters, and a few personal effects. The views from the roof over Dublin Bay are excellent.
From the tower you look down on the Forty Foot, a tidal swimming hole cut into the rocks below that has been in continuous use for over 200 years. Open to all (it was men-only until the 1970s). Cold. Year-round.