The modern boundary fence running across the east-facing pasture takes an unexplained bend, and that bend, visible on Ordnance Survey maps as far back as 1842 and again on the 1905 edition, is all that was left to mark the outline of a ringfort that had otherwise been levelled into the surrounding farmland.

A ringfort, or rath, is a roughly circular enclosure defined by earthen banks and ditches, built predominantly during the early medieval period in Ireland as a farmstead or place of shelter. Most are recognised today by the raised ring they leave on the landscape; here, the levelling has been thorough enough that the old boundary had to do the work of memory instead. An arc of earthen bank, nearly thirty metres long, does survive on the eastern to south-western side, incorporated into the existing field boundary system rather than demolished outright. Its inner face is stone-faced, constructed in a style consistent with the dry-stone field fences around it, which may explain why it was absorbed rather than cleared. The bank stands around 1.1 metres above the interior and 1.6 metres above the exterior, with a shallow external fosse, a ditch, surviving at the south-western end to a depth of about 0.3 metres. The full enclosure would have measured roughly 35 metres on its longer axis and 25 on its shorter, and a faint trace of a low rise suggests where the remainder of the interior boundary once ran, though its height can no longer be measured with confidence.

What makes Cloonsillagh quietly instructive is how the fort survived at all. Levelled ringforts are common across Cork and the rest of Ireland, casualties of centuries of agricultural improvement, but this one left its ghost in the administrative geometry of the landscape. Townland and barony boundaries, which were fixed long ago and rarely adjusted, occasionally preserve the outline of features that have since disappeared from the ground, fossilising the shape of an enclosure that farming otherwise erased.