Dublin's most important medieval port has long since settled into its role as an elegant dormitory village, but there are some revealing vestiges of its illustrious past, most notably the remains of three of the eight castles that once lorded over the area.
Dalkey is small enough that you can get around on foot. Most visitors will be arriving by DART, so start your exploration in the middle of town: the main sights are on Castle St, as are most of the cafes (or on the streets just off it). Coliemore Harbour is where you can get boat trips; overlooking the adjoining Bullock Harbour are the remains of Bullock Castle.
The waters around the island are popular with scuba divers; qualified divers can rent gear in Dun Laoghaire, further north, from Ocean Divers.