Known locally as Dún Fearbhaí or simply Mothar, this early Irish cashel, a type of dry-stone fortified enclosure, occupies a sheltered position on the lee side of the island's peak, tucked away from the Atlantic wind that defines everything else on this exposed limestone plateau.

The structure is subrectangular in plan, measuring roughly 27 metres in length and nearly 24 metres wide, and it survives in notably good condition. Its defining wall is built in the dry-stone tradition, meaning no mortar, just carefully fitted stone, and the interior retains a terrace running along the wall face, connected by a series of steps that allowed movement between levels. The entrance faces east. Antiquarians were documenting it as far back as the 1880s and 1890s, with T. J. Westropp recording it in 1895, and O'Flanagan covering it in some detail in the 1920s. That record of scholarly attention speaks to a structure that has always rewarded close inspection, even if it sits in the shadow of better-known Aran monuments.

Inhis Meáin is the least visited of the three Aran Islands, which means Dún Fearbhaí sees comparatively little foot traffic despite its protected status as a National Monument. The approach, up toward the highest ground of the island, is part of the experience; once at the site, the interconnecting steps and internal terrace give a clearer sense of how such enclosures functioned as more than simple walls, with their raised walkways suggesting a concern for both defence and internal organisation.