It measures roughly 40 metres in external diameter, its bank a modest 0.6 metres wide, and one end is open rather than closed, which already sets it apart from the more familiar circular enclosures of the Irish countryside. Dense vegetation has done much of the work of concealing it, and without prior knowledge of its existence, there is very little to catch the eye.

The enclosure appears on the Ordnance Survey six-inch map of 1843, which means it was visible and mappable in the nineteenth century, even if its origins stretch back considerably further. Compilers Geraldine Stout and Padraig Clancy recorded it as part of a broader survey of Dublin's archaeological landscape, and their notes place it in the context of a nearby ringfort to the south-east. A ringfort, for those unfamiliar with the term, is a roughly circular enclosure defined by earthen banks or stone walls, typically associated with early medieval farmsteads in Ireland. The proximity of the two features suggests this part of Three Rock Mountain was not simply wild upland but a place where people organised space, demarcated land, and perhaps sheltered animals or stored goods over a long period. Whether the open-ended enclosure is contemporary with the ringfort, earlier, or later, the notes do not specify.

Three Rock Mountain sits within the Dublin Mountains and is accessible from several points in the foothills south of the city. The forestry plantation that covers this part of the summit makes navigation a matter of patience; tracks run through the trees but the understorey is thick, and the low earthen bank of the enclosure is unlikely to announce itself clearly. Visiting outside the summer months, when vegetation dies back somewhat, may improve visibility. The site itself is not marked or interpreted on the ground, so bringing the relevant OS map sheet and cross-referencing with recorded coordinates is advisable. The ringfort to the south-east offers a secondary point of interest once the enclosure has been located, and together they give a quiet sense of how layered even an apparently empty hilltop can be.