A promontory fort uses the natural advantage of a narrow neck of land to reduce the amount of artificial fortification required, closing off a headland with walls and ditches so that the sea does the defensive work on the remaining sides. Here, though, the scale and layering of the defences goes well beyond the simple. Three or four lines of walls and banks, separated by level spaces and shallow fosses, step down the landward approach across a stretch of roughly 200 metres, isolating an interior area of approximately 350 by 320 metres. That interior is not flat farmland but a steep, rocky ridge running northeast to southwest and rising to 115 metres above sea level, its slopes falling sharply to the cliffs below.

What makes the site at An Coimín particularly strange is not the outer defences but what fills the enclosed space. Along the ridge and its flanks, at least fourteen hut-sites survive, ranging from sub-rectangular to circular in plan, their walls built partly from drystone and partly from the outcropping bedrock that breaks through the hillside at intervals. Several are conjoined, suggesting a settlement of some density rather than isolated shelters. But the more puzzling feature is a series of at least 75 shallow depressions cut into the hillside along the three seaward sides of the promontory, running in a continuous and apparently deliberate sequence along the upper edge of the scarp and cliffs. They are no more than 75 centimetres deep, between one and five metres across, and contain no masonry whatsoever. Two show traces of entrance gaps and enclosing banks, which might suggest ruined hut-sites of a very slight construction, but the function of the rest is genuinely unclear. A comparable set of depressions has been noted at the nearby promontory fort at Ballyoughteragh South, which deepens rather than resolves the puzzle. The main defences themselves have been considerably disturbed over the centuries; stone was robbed to build sheep-shelters along the inner wall, and one of these shelters was constructed inside an earlier circular hut roughly 6.5 metres in internal diameter. Three entrance gaps exist in the defences, the two northernmost now blocked by later walling, with a causeway on the southern side extending only as far as the berm between the outer banks and the inner wall. The description of the site was first set out in detail by J. Cuppage in the 1986 Dingle Peninsula archaeological survey, published by Oidhreacht Chorca Dhuibhne.