This circular stone enclosure, measuring approximately 21.5 metres in internal diameter, overlooks the landscape to the north whilst being sheltered by higher ground to the south. The structure consists of a low bank built from stones mixed with earth, though time and human activity have taken their toll on its original form.

The cashel’s preservation varies considerably around its perimeter. Whilst portions of the wall remain visible to the south and west, the northwestern and northern sections have disappeared entirely over the centuries. The eastern side has suffered a different fate; here, a road cuts directly through what would have been the defensive wall, running tangent to the original circular plan. Archaeological surveys suggest there may have been an entrance in the southwestern quadrant, though this interpretation remains tentative given the structure’s current condition.

This site represents one of many ringforts scattered across the Irish countryside, defensive homesteads that once housed farming families during the early medieval period. The Archaeological Survey of County Donegal, compiled by Brian Lacey and his team in 1983, documented this particular cashel as part of their comprehensive catalogue of the county’s field antiquities, which spans from the Mesolithic period through to the 17th century. Despite its partial destruction, the Aghaglassan cashel continues to mark the landscape as a tangible link to Ireland’s early medieval past.