The example at Castlecarra, on the shores of Lough Carra in County Mayo, applies exactly the same logic to an inland landscape. Here, a narrow neck of land leads out into what is now Doon Wood, a sprawling area of trees and scrub, and it is across this neck that an impressive wall of dry-stone masonry runs from one side to the other, sealing off the promontory entirely.

The wall is substantial by any measure: 114 metres long, 15 metres wide, and still standing to a height of 2.2 metres in places. It stretches the full width of the neck, leaving no way through except a gap of roughly 3 metres near the centre, which is thought to be a considerably later addition rather than the original entrance. A field fence now runs along the top of the structure, a common fate for ancient stonework in the Irish countryside, where later agricultural boundaries frequently made use of whatever already stood. At the western end of the enclosed area, close to the lakeshore, two further enclosures have been identified, suggesting the defended space was not simply left empty but contained internal organisation of some kind. The combination of the lakeshore edge and the blocking wall would have made the promontory a remarkably secure position, its builders borrowing from the sea-cliff logic of coastal fort construction and applying it to the quieter geography of a Connacht lakeshore.