Cartographers who came before the age of aerial photography looked at the earthen bank cutting across a headland on the Mayo coast near Achadh Ghlaisín and mapped it as a field wall. It took a view from the air to reveal what was actually there: a promontory fort, a type of defended enclosure in which a natural headland does much of the defensive work, with a constructed barrier sealing off the landward approach. The site had simply never been recorded before that aerial observation.

The headland itself is roughly triangular, measuring 44 metres in both length and width, and sits above steep, jagged cliffs that make the sea below completely inaccessible from the fort. A single straight bank and fosse, the fosse being the ditch dug alongside a defensive bank, run from one cliff edge to the other, cutting the headland off from the mainland. The round-topped earthen bank is 1.6 metres wide, and the flat-bottomed fosse beside it is the same width, sitting about 0.9 metres below the top of the bank. The defences are notably consistent in their dimensions along their entire length. Visible from the air, though difficult to make out on the ground, is a stone-lined causeway placed centrally across the fosse, almost certainly the original entrance point. There is no counterscarp bank, meaning the outer lip of the fosse was left unraised. The boggy interior, undulating and featureless as it now appears, may well be concealing whatever structures once stood inside, the blanket bog that covers the site and surrounding hillside a slow but thorough obscurer of evidence. The detailed description of the fort comes from a 1999 MA thesis by Markus Casey, which surveyed coastal promontory forts across counties Sligo, Mayo, Galway, and Clare.