Dunnarealt Island, accessible by foot when the sea retreats, hosts a cashel that occupies the entire landmass; a 19-metre diameter defensive structure ingeniously built into the natural rock platform. The fort’s builders worked with the island’s geology, incorporating rocky outcrops into collapsed stone walls that ring the site. These walls, constructed from large boulders with cores of smaller stones, appear deceptively low from within, whilst gaps between the wall and platform have been carefully packed with stones and clay to create a continuous defensive barrier.

Within this ancient fortification lie the remains of two rectangular stone buildings in the northeastern section, their walls still traceable despite centuries of weathering. The larger structure measures 7.5 metres externally and 5.55 metres internally from north to south, with east to west dimensions of 5.7 metres externally and 4.55 metres internally. A second, more modest building adjoins the northwest corner of the first, though vegetation has claimed much of it; this structure spans 5.6 metres externally and 3.25 metres internally on its north-south axis, whilst measuring 5.4 metres externally and 3.2 metres internally from east to west. These buildings likely once connected directly to the cashel wall, forming an integrated defensive complex.

The fort’s strategic position, becoming an island fortress at high tide and a mainland stronghold at low tide, speaks to the sophisticated understanding its builders had of both defensive architecture and tidal patterns. This dual nature would have provided both security and accessibility, allowing inhabitants to control access whilst maintaining connections to the mainland when needed. The site represents one of many such coastal fortifications scattered across Donegal’s rugged shoreline, each adapted to its unique geographical setting.