A fairly substantial one, in fact, roughly 70 metres across in its interior, semicircular in plan, with traces of hut-sites within its boundary and a separate ring of stones about 20 metres in diameter sitting some 35 metres to the north. The problem is that nobody has been able to find it. Repeated field walking across the boulder-strewn, heath-covered ground has failed to identify any of these features, leaving a peculiar gap between the cartographic record and whatever is actually on the hillside.

The enclosure first appears on the Ordnance Survey six-inch map of 1843, one of the earliest systematic surveys of the Irish landscape and a document that has guided archaeologists and historians ever since. The surveyors who worked this stretch of the Dublin uplands marked not only the main semicircular enclosure but also two associated hut-sites within it and a smaller stone ring with a south-east annexe to the north. Hut-sites of this kind are typically the collapsed or overgrown remains of small circular or oval structures, often associated with early medieval settlement or seasonal pastoral activity in upland areas. Whether the original surveyors recorded something that has since been entirely obscured by vegetation and peat growth, or whether they were working from earlier sources of varying reliability, is not clear. The site record was compiled by archaeologists Geraldine Stout and Padraig Clancy and uploaded in July 2018, with the absence of ground evidence noted plainly.

The terrain here is rough and open, the kind of Dublin mountain landscape where paths can dissolve into boggy ground without much warning. Two Rock Mountain is accessible from several directions, and the broader area sees reasonable foot traffic from hillwalkers, but the specific slope above Ballybrack where this enclosure is recorded offers little in the way of waymarked routes. Anyone drawn to investigate should be prepared for uneven, boulder-scattered ground and heath that can obscure low features entirely. There is a certain strange pleasure in arriving at a grid reference where something significant was once mapped and finding only sky, stone, and silence.