This is a ring barrow, a form of prehistoric funerary monument in which a burial mound is encircled by a fosse, or cut ditch, sometimes accompanied by an outer earthen bank. The example at Cuillaun is modest by any measure: the central platform rises no more than 0.7 metres at its highest point and measures roughly six metres across, while the fosse enclosing it is just over a metre wide and barely a quarter of a metre deep. What makes it quietly interesting is precisely this smallness and its setting, caught between a steep ridge to the north and the lake shore to the south.

The monument retains a faint suggestion of an external bank on its south-eastern edge, a low rise that may represent what once formed a more complete enclosure around the fosse. That outer feature has been eroded or disturbed elsewhere, and on the north-north-west side the fosse itself is interrupted by a shallow rectangular pit running up against a farm lane. A small oval quarry pit sits about 1.5 metres to the east, suggesting the ground nearby has been worked at some point, likely drawing material from the vicinity of the monument. A second ring barrow of the same class lies roughly 140 metres to the south, closer to the edge of Lough Killaturly, meaning this particular stretch of Mayo lakeside once held at least two such burial markers within clear sight of one another.