Roughly twenty metres in diameter, it belongs to a category of prehistoric or early medieval feature found across Ireland, typically a roughly circular area defined by an earthen bank or ditch, possibly once used for settlement, agriculture, or ritual. For most of the year it is simply gone, swallowed by the reservoir that now dominates this part of County Wicklow.

The enclosure was recorded on the Ordnance Survey six-inch map of 1838, which means surveyors could still see it clearly on the ground at that point. Blessington Reservoir, also known as Poulaphouca Reservoir, was created in the early 1940s when the River Liffey was dammed to generate hydroelectric power, flooding a significant stretch of farmland and a number of townlands, Ballyknockan among the areas affected by the rising waters. Whatever the enclosure once was, field monument or remnant of an earlier settled landscape, it passed from view when the valley flooded. The 1838 map reference is now the clearest evidence that it existed at all as a visible feature.

In dry summers, when water levels drop sufficiently, the site is occasionally exposed, though even then there is nothing to see at ground level. The enclosure does not announce itself. What remains is essentially cartographic, a circle on an old map corresponding to a patch of lakebed that, under the right conditions, briefly becomes land again.