Known locally as Doonroe, or An Dún Rua in Irish, this is a promontory fort, a type of coastal defensive enclosure common along Ireland's Atlantic seaboard, where the sea itself does much of the defensive work and earthworks are needed only on the landward side. What makes Doonroe quietly compelling is the precision with which it was engineered for that purpose, and the degree to which that engineering still reads in the landscape.

The fort measures 74 metres across at its landward end and extends roughly 58 metres toward the cliff edge, its interior rising gradually before dropping sharply at the northern margin. The landward defences consist of two parallel banks separated by rock-cut fosses, which are ditches cut directly into the bedrock, with a narrow causeway traversing them to form the entrance. When the antiquarian Thomas Johnson Westropp surveyed the site in 1912, he noted that the outer fosse was approximately five feet deep and that the inner bank still carried the remains of a drystone wall along its crest. Both features have since deteriorated: the fosse now varies between roughly half a metre and one metre in depth, and no trace of the wall survives. At the inner end of the causeway, at least six large flat slabs lie prostrate on the ground. Westropp believed these once stood upright, forming the sides of a formal entrance passage or gateway. One substantial slab remains set on edge within the inner fosse. Inside the enclosure, a pair of conjoined huts is also visible, their outlines preserved in the turf.

The western section of the defences survives in the best condition, and it is here that the double-bank arrangement is most legible, the banks still rising to a maximum height of around 2.6 metres above the base of the inner fosse. The causeway itself is 12.8 metres long and only 2 metres wide, a deliberately narrow approach that would have made any entrance slow and controlled. Whether the fallen gateway slabs toppled through neglect or deliberate clearance is not recorded, but their presence, scattered and flat where they once stood tall, gives the entrance area a slightly unsettled quality that the surrounding sea cliffs only amplify.