A roughly rectangular structure sits about midway along the fort's interior platform, its foundation lines still legible beneath the turf, its external corners softly rounded rather than angular. The enclosing bank, reaching no more than half a metre in height, runs along its perimeter with a mane-like ridge of grass along the top, giving the whole thing a slightly shaggy presence on the ground. Gaps in the bank near the north-western and south-western corners suggest it was never entirely complete, or at least that something disrupted it over the centuries. The original entrance, a modest opening of about 0.8 metres wide, appears to have faced east-south-east.

When the antiquarian Thomas Johnson Westropp visited in the early twentieth century, recording the archaeology of Clare Island as part of a broader survey, he noted this structure in 1911 and again in 1914 with characteristic economy, calling it simply "another house." The phrase is telling in its casualness. It sits only three metres south of a second house within the same fort enclosure, and neither building has been dated with any precision. A promontory fort is a defensive enclosure that uses a natural headland as its boundary on two or more sides, with an artificial bank or ditch cutting across the landward approach. Doon is one such site, and these domestic structures within its platform suggest that the fort served as more than a temporary refuge, though exactly when, and for how long, remains open.

The interior of the structure is uneven underfoot, the ground having settled and shifted in ways that make the space feel genuinely old rather than reconstructed. The dimensions, roughly 7.55 metres along the longer axis and 4.8 metres across, are modest but not insubstantial for a vernacular building of uncertain date. It is the kind of place that rewards patience and a willingness to read the landscape slowly.