It is easy to miss, not least because the ground it encloses is mostly bare outcropping rock and coarse vegetation, and the earthworks have long since lost their original height. Yet this modest depression is all that remains of a promontory fort, a type of coastal enclosure in which a natural headland does most of the defensive work, with a fosse, essentially a ditch, cut across the landward side to complete the barrier. The headland itself was once considerably larger; sea rocks extending some 100 metres from the site suggest the shoreline has retreated significantly since the fort was in use.

When the archaeologist Liversage excavated the fosse in 1957, publishing the results in 1968, the findings complicated what looked on the surface like a simple earthwork. The ditch had originally been steep-sided, five metres wide and over two metres deep, far more substantial than what survives above ground today. Its fill contained animal bones and iron slag, hinting at activity, possibly metalworking, in or near the enclosed area. The material dug from the fosse had been used to construct an external bank measuring eight and a half metres wide and nearly a metre and a half high. A slight gap near the north-western edge of the fosse has been interpreted as a possible causewayed entrance, where a raised crossing would have allowed controlled access. The site sits within a wider landscape of layered history: a seventh-century church dedicated to St. Begnet stands on the northern slopes nearby, and the higher ground to the south is occupied by a Martello tower, the squat circular fortifications built along the Irish coast during the Napoleonic period, with a gun emplacement added on the southern shore at much the same time.

Dalkey Island is reachable by a short boat crossing from Coliemore Harbour in the Dublin suburb of Dalkey, with seasonal ferry services typically running during the warmer months. The fort itself occupies the north-western corner of the island, a compact area of roughly a quarter of a hectare divided into two roughly flat platforms. The fosse is most visible along the eastern side, where it retains the most depth. The interior offers no shelter and no obvious path, so flat-soled footwear and some tolerance for uneven rock are useful. The proximity of St. Begnet's church means the two sites can be taken in together without much effort.