Known locally as "The Caher", a caher being a type of drystone ringfort more commonly associated with the west of Ireland, it sits on fairly level ground near Reefert Church and presents walls of between 2.4 and 3 metres thick, built without mortar and carefully fitted in a manner that one mid-twentieth century observer compared directly to the cahers of Kerry and Clare. Its external diameter runs to about 20 metres, and on the eastern face, four or five small stone flags are set vertically into the wall, their purpose unexplained.

The structure has attracted puzzlement since at least the Ordnance Survey Letters of 1838 to 1840, when it was noted that two enclosures near Reefert were not of the type "vulgarly called Danes forts" but were more probably enclosures for cattle. The description points to a long-standing uncertainty: is this a genuine early medieval cashel, that is, a stone-walled ringfort of the kind that would have enclosed a farmstead or small settlement, or something else entirely? Harold Leask recorded it in 1950 as dilapidated and tree-grown, with no obvious entrance except a gap to the south-east. When D. Sweetman carried out limited excavation in 1983, no early features were found, raising the uncomfortable possibility that the structure, or at least the form it now takes, may be modern, perhaps a later rebuild or modification of whatever originally stood here.

The south-east entrance gap, now widened to around 2.2 metres, and the boulders and upright flagstones lining the inner face along the northern, eastern, and southern sides are the details worth examining closely on a visit. The site lies within the Glendalough valley and is a National Monument in state ownership, accessible on foot along the south shore of the Upper Lake.