Cathair Coinn is remarkable for what does not. The cashel, or stone-walled ringfort, that once occupied a gentle north-facing slope overlooking Smerwick Harbour on the Dingle Peninsula has vanished so completely that it left no visible trace on the ground. It was known well enough to be recorded on the Fair Plan, the pre-publication survey drawings used in the preparation of the Ordnance Survey maps in the nineteenth century, where it appeared under the name 'Cahir (site of)', already acknowledged as a ruin. Yet even that cautious designation was dropped before the maps went to press, and the site disappeared from the published editions entirely.

The Ordnance Survey Name Book described it as a circular earthen fort roughly two chains, or about forty metres, in diameter, and noted that it had already been levelled to the ground by the time of recording. A cashel is typically defined by a roughly circular enclosure of dry-stone walling, used in early medieval Ireland to demarcate a farmstead or the residence of a person of some local standing. Whether Cathair Coinn ever had stone walls to speak of, or was always primarily an earthen construction, cannot now be determined. What can be said is that in 1966, a souterrain was discovered approximately one hundred metres to the north-north-west of the fort's recorded position. A souterrain is an underground passage or chamber, usually stone-lined, associated with early medieval settlement sites and thought to have served as storage space or a place of refuge. Its proximity to Cathair Coinn strongly suggests the two features were connected, part of the same small settlement economy that once looked out across the harbour below.