Doon, Inis Gé Thuaidh, Co. Mayo

That ambiguity is not a failure of research so much as a reflection of how remote and difficult this place genuinely is. The promontory in question extends from Iniskea North, one of the Iniskea Islands, separated from it by a narrow tidal sea channel. Whether what survives there constitutes a promontory fort, the kind of coastal defensive enclosure that uses natural cliff edges in place of walls on its exposed sides, has been a matter of quiet scholarly disagreement for over a century.

The antiquarian Thomas Johnson Westropp noted the promontory, which carries the name Doon, a word derived from the Irish dún meaning fort or enclosed place, in 1914. He never visited the island himself, but recorded his understanding that the structure resembled examples he knew from counties Clare and Kerry, describing it as a rock platform rising from tidal reefs and walled. When Casey returned to the question in 1999, listing it as a possible promontory fort, aerial reconnaissance failed to confirm the presence of any such structure. The matter might have rested there, except that a drone survey carried out in August 2016 recorded the remnants of an enclosing stone rampart, partially obscured by storm debris. The evidence, then, is real but fragmentary, the kind of survival that the Atlantic coastline specialises in producing.