The result is one of the more quietly logical defensive sites in Kerry. Lough Adoon sits in a corrie, the kind of steep-walled hollow scooped out by glacial action high in the Ballysitteragh-Beenoskee mountain range on the Dingle Peninsula, and Dooneen Island occupies its centre. To the east, a shallow ford no more than thirty metres across and barely ankle-deep connects the island to the shore. Everything else is sheer drop and cold water.

A cashel is a stone-walled ringfort, the drystone equivalent of the earthen raths found across much of Ireland, and the one on Dooneen Island was built to exploit its geography with some precision. Rather than encircle the entire island, the builders raised a drystone wall along the eastern arc alone, following the natural curve of the shoreline for roughly fifty metres and trailing off into the water at both ends. That wall still stands for most of its length, reaching up to two metres in height and averaging two and a half metres in width, with a centrally placed entrance just under one and a half metres wide. A short stretch of wall-facing survives on the southern side of the island, hinting that the defences may once have extended a little further, but there is no evidence that the rest of the perimeter was ever enclosed. The site was recorded in J. Cuppage's 1986 archaeological survey of the Corca Dhuibhne region.

The approach from the east is genuinely straightforward once you have reached the lake, with the water shallow enough to wade. Once on the island, the entrance gap in the wall is easy to locate at the centre of the arc, and about eight metres north of it there is a small rough recess set into the inner face, though its purpose is uncertain and it may not be an original element of the structure. The wall-footing along the same stretch projects slightly beyond the inner face for a distance of six metres, a detail that suggests the builders were thinking carefully about how this particular section would perform.