At its narrowest point, roughly 30 metres across, a stone wall cuts off the northern portion of the island. That wall is the reason the place is of any particular interest, because what it may represent is a coastal promontory fort, a type of enclosure in which a wall or bank is thrown across a headland or narrow neck of land to create a defended space, using the sea itself as the remaining barrier on all sides. The site is not marked on the Ordnance Survey maps, and the island is currently inaccessible.

The wall is visible from the air, though no other structural features have been identified on the ground. The site's status remains uncertain, which is part of what makes it quietly compelling. The antiquarian T. J. Westropp, writing in 1912, noted that he had heard the island was walled, but concluded the feature was probably natural rather than man-made. That scepticism has lingered. It was only through the later work of Markus Casey, whose 1999 thesis surveyed coastal promontory forts across Sligo, Mayo, Galway and Clare, that the site was formally considered as a possible fortification. Casey's research also noted that the island was once probably joined to the mainland, which would have made such a construction far more practicable, a wall across a narrow ridge creating a fortified enclosure at the tip of what was then a peninsula rather than a detached island.