Dún Formna occupies the summit of a low terraced limestone hillock on Inis Oírr, its oval cashel wall, a cashel being a stone-built ringfort, tracing the natural edge of the limestone scarp rather than cutting across it. The enclosure measures roughly 52 metres north to south and 41 metres east to west, and the drystone wall that defines it is described as massive, following the rock's own contour as though the builders were working with the island rather than simply upon it.

The site layers its periods visibly. A medieval castle stands inside the cashel's interior, and a small rectangular turret projecting from the south-west corner is believed to be associated with it, suggesting the earlier enclosure was adapted and pressed into service centuries after it was first raised. Nineteenth-century observers recorded a number of possible clochans within the interior as well; clochans are small dry-stone beehive huts associated with early medieval occupation, though no trace of them survives today. The entrance to the cashel faces north-east. By the time Westropp wrote about the site in 1895, and O'Flanagan and Mason visited in subsequent decades, the structure had already undergone considerable restoration, a fact acknowledged openly in the archaeological record.

The hillock overlooks the beach, which means the fort commands a clear view over the island's eastern shore. The much-restored state of the walls is worth keeping in mind when looking at the stonework; what a visitor sees today reflects both ancient construction and later intervention, the two not always easy to distinguish from one another.