This is Dundagallan, or Dún Dá Ghallán in Irish, a promontory fort of the kind once used, most likely in the Iron Age, to enclose and defend a narrow headland by blocking its only accessible landward approach. What makes this one particularly striking is not the fort's interior, which survives as a flat and featureless grass platform, but its entrance: two tall standing portal stones, set less than a metre apart, still upright on the outer face of the defensive bank. They frame a causeway barely two metres wide, crossing the fosse, the rock-cut ditch that separates the promontory from solid ground.

The portal stones themselves are impressive in their precision. The southern slab stands nearly two metres high, the northern one slightly shorter at 1.68 metres, and both are orientated with their long axes running perpendicular to the line of the defences, suggesting they were part of a deliberately designed entrance structure rather than incidental stonework. The fosse is cut in two sections, one extending from the south-east cliff edge and another, less well defined, from the north-west, with the narrowest point of the headland determining where the earthwork sits. When the antiquarian T. J. Westropp visited and described the site in 1912, he noted an unusual holed stone lying in the base of the fosse, a slab roughly one and a half metres long with a circular perforation at one end. Its purpose was not recorded, and by the time later surveys were carried out, it had vanished entirely, leaving only Westropp's measurements as evidence it ever existed.

The promontory projects about 60 metres out to sea and reaches only 12 metres at its widest point, so the fort itself was never large. The cliffs on either side would have made the single defended landward neck more than sufficient. The two portal stones remain the most legible feature on the ground, and they repay close attention: the care taken to erect them, at a site where the sea does most of the defensive work, implies an entrance that was meant to be seen as well as crossed.