At the northern extremity of the Island there is evidence of a deep ditch or fosse that was the boundary of a medieval promontory fort.

Dalkey Island, only 5 minutes by boat from Coliemore Harbour, is an important site of ancient and historic remains.

[7] It was an important site of pilgrimage in the medieval period as evidenced by the prominence of the island on early maps.

One such, Danti's map of 'Isole Britaniche', painted on the wall of the Palazzo Vecchio in Florence (c. 1560) survives as a remarkable record.

Recent tests of the water show that it has high levels of vitamin C (the cure for scurvy), probably due to its position below a salt marsh.

Another church in nearby Dalkey also named for St Begnet may have been a 'base' for pilgrims before they crossed the sound to the island.

'Pilgrim badges' would have been sold in the town which would be eventually sewn into their burial shroud to prove that they had undertaken the pilgrimage, helping their journey through the gates of heaven.

The church on the island was altered during this period on the east side, when builders used it as living quarters while building the nearby Martello tower and gun battery in 1804.

[8] Extensive archaeological excavations took place in the late 1960s (Liversage) which uncovered evidence of a churchyard and a number of burials.

The skeletons were not exclusively male, as has been suggested but they were in the majority in keeping with the fact that it was an early Christian Monastic site.

Coibhdeanagh, the Abbot of Cill-achaidh, drowned in the sea off the island trying to escape with treasures from the church hidden in his vestments during one Viking raid.

According to Samuel Lewis's 1837 Topographical Dictionary of Ireland, "[t]radition states" that Dalkey was used as a place of refuge for some citizens of Dublin during the "great plague" of 1775.

The beacon stands on the site of what was once a 'gibbet' where the bodies of two pirates, Peter McKinlie and George Gidley were displayed following their execution for the murders of Captain Cockeran, Captain George Glas and his family and others on board the ship Earl of Sandwich in 1765.