Hut site, Acaill Bheag, Co. Mayo

A promontory fort is a coastal enclosure where natural cliffs or the sea itself form most of the perimeter, with an earthwork or stone barrier cutting off the landward side. The hut's integration into that barrier is what makes it architecturally curious; rather than standing freely within the enclosed space, it was constructed as part of the defensive fabric itself.

The structure was first recorded by the antiquarian Thomas Johnson Westropp, who visited in the early twentieth century and noted an oval hut measuring roughly 8 metres north to south and 10 metres east to west, with walls somewhere between 2 and 3 metres thick. Those are substantial walls, suggesting a building meant to last, or perhaps to serve a purpose beyond simple shelter. When the site was revisited by Casey in 1999, the picture had shifted somewhat: what was observed was a low circular structure with an internal diameter of around 9 metres, and a scatter of slabs across the interior pointing to stone construction throughout. The slight discrepancy between Westropp's oval and Casey's circle may reflect the gradual collapse and spread of the walling over the intervening decades, or simply a difference in how each recorder interpreted what remained on the ground.