Whatever the feature once was, or was believed to be, it was large enough to record: the outer circle suggests a maximum diameter of around 36 metres. By the time later editions of the same map series were produced, the symbol had vanished, and when the site was examined on the ground in 1990, no visible trace of any earthwork remained.

That disappearance is worth pausing over. Mounds in the Irish landscape take many forms, from prehistoric burial mounds to Norman mottes, the flat-topped earthen platforms built by Anglo-Norman lords to carry timber or stone towers, to much more modest and ambiguous earthworks whose origins were never recorded. The 1838 survey was compiled at a moment when rural Ireland was being mapped with unusual care and detail, and the surveyors occasionally noted features that local tradition identified as significant, even when their precise nature was uncertain. Whether the Clonwilliam feature was a genuine prehistoric or medieval earthwork, a natural rise in the land, or simply an error of interpretation, is now impossible to say. Agriculture, drainage works, and the general reshaping of farmland across the nineteenth and twentieth centuries account for the loss of countless such features across the country, and Clonwilliam appears to be one more quiet casualty of that process.