Dalkey Island is an uninhabited island about 16 km (10 mi) south of Dublin, near the village of Dalkey, 3 km (2 mi) south of Dún Laoghaire harbour. Its name is a meld of the Irish deilg ("thorn") and the Old Norse øy ("island").The island is now uninhabited by humans, but there are the remains of houses, a church and a Martello Tower. Located less than 300 metres offshore the island comprises 9 hectares (22 acres). From Bullock Harbour, it is possible to rent a boat to visit the island, and also from Coliemore Harbour.

Dalkey Island, only 5 minutes by boat from Coliemore and Bulloch Harbours, is an important site of ancient and historic remains. Artefacts from the island, now housed in the National Museum in Dublin, are evidence that the original occupants were from the Mesolithic or Middle Stone Age. Settlers continued to use the site through the Iron Age and Early Christian period.

There is evidence it was inhabited in the 4th millennium BC (6,000 years ago) and was also used as a Viking base. There are ruins of another church, dating from the 7th century, named after St Begnet. This was altered on the east side when builders used it as living quarters while building the nearby Martello tower and gun battery in 1804. An older wooden church was probably here before the present stone one was built.

A promontory fort was located at the northern end of the island, its presence still visible today in the form of a ditch. A herd of goats, originally put there in the early 19th century, remains there today but they are replacements of the original goats which were removed.
Dalkey Island, with Sorrento Point to the left, The Muglins to the right. Howth peninsula is in the background

The ruined stone church was built in the 9th/10th century and was probably abandoned when the Vikings used the island as a base to form part of the busiest port in the country at that time. In the early 19th century the British Admiralty erected the Martello Tower, one of eight dotted along the Dun Laoghaire coastline, as an early warning defensive device against the one time threat of invasion during the Napoleonic Wars.