Doondoillroe appears on every edition of the Ordnance Survey six-inch maps, its name marking a west-facing headland roughly 170 metres long and 45 metres wide. A promontory fort is a form of ancient enclosure in which natural cliff edges do the defensive work on most sides, with earthen banks and a fosse, meaning a ditch, cut across the landward approach to seal off the promontory. At Doondoillroe, those earthworks crossed the headland roughly north to south, with a causewayed entrance running through the middle. What is unusual here is not the form of the monument but the speed of its erasure.

When the antiquarian T. J. Westropp described and illustrated the site in 1908, the defences were sufficiently legible that he could map them in some detail, even noting a path down to the sea just west of the embankments. When Markus Casey visited in 1999, the remains still closely matched Westropp's account. The inner bank stood at an internal height of 2.65 metres and was nearly ten and a half metres wide, built of sand, gravel, and earth. The flat-bottomed fosse beside it was nearly five metres deep. A second, outer bank ran parallel to it. Some earlier quarrying had already removed the southern portions of both banks, but the core of the monument was intact. By 2003, on a return inspection, all of it had gone. Quarrying for shale and gravel had cut to depths of up to ten metres across most of the headland. What survives is a grassy platform about 20 metres long at the western tip of the promontory, two shallow drainage channels, and a scraped expanse of bare ground to the east where the fortifications once stood. Three ring-barrows, low circular burial mounds, lay in the field immediately to the north; one of those is now either quarried away or buried under spoil.