The fort's landward side, measuring 17 metres across, is protected by an earthen bank that rises 0.8 metres high, accompanied by a flat-bottomed ditch, or fosse, that runs 2.8 metres wide and plunges up to 1.5 metres deep. Two stones found within the fosse hint that it may once have been lined with stone revetting to prevent erosion and strengthen the defences. Curiously, whilst a causeway 1.8 metres wide crosses the fosse slightly southwest of centre, there's no corresponding break in the defensive bank above it, suggesting a more complex entrance arrangement than first appears.

The interior of the fort reveals further defensive planning, with a low bank just 0.3 metres high running around most of the promontory's edge, positioned 2 to 3 metres back from the cliff face. This inner bank encloses an almost rectangular space measuring 12 metres east to west and 8 metres north to south, though it fades away on the western side where natural defences may have been deemed sufficient. The entire promontory extends 15 metres out from the mainland, creating a naturally defensible position enhanced by human engineering.

This coastal fortification represents one of many promontory forts that dot Ireland's Atlantic coastline, where communities once used the combination of sheer cliffs and constructed earthworks to create defensible spaces. The site was documented in the Archaeological Survey of County Donegal, compiled by Brian Lacey and his team in 1983, which catalogued field antiquities from the Mesolithic period through to the 17th century. Today, An Uillinn stands as a testament to the ingenuity of those who recognised the defensive potential of Aranmore's dramatic clifftop geography.