At Killakee, on the lower slopes of Mountpelier Hill in County Dublin, there is an enclosure that leaves no mark on the ground whatsoever. No earthwork, no ditch, no raised edge. The only evidence for it comes from a single aerial photograph, taken in 1963, in which the outline of a roughly circular form emerges from the cropmarks or soil variations that cameras occasionally catch where human eyes cannot.

The enclosure was recorded and estimated at approximately 60 metres in diameter by Healy in 1975, a measurement derived from that aerial image rather than any ground survey. It sits within a forest clearing, downslope from Mountpelier Hill, the prominent summit known for the Hell Fire Club hunting lodge built in the 1720s. Enclosures of this general type, large circular features defined by a bank or ditch, are associated across Ireland with a wide range of periods and purposes, from prehistoric settlement and ritual use to early medieval farming activity. Without excavation, it is impossible to say which category this one belongs to. What can be said is that a standing stone, a single upright cut stone placed deliberately in the landscape, survives to the west of the site, catalogued separately in the national record. Standing stones frequently appear in proximity to enclosures and burial monuments, suggesting the two features may share a broad chronological or ceremonial relationship, though again, no specific connection has been established here.

For anyone visiting the Killakee area, the forest roads and trails below Mountpelier Hill are accessible from the Hellfire Club car park off the Killakee Road. The enclosure itself offers nothing to see at ground level; the standing stone to the west is the tangible thing worth locating, though it requires attention to the landscape to pick out. The clearing where the enclosure was photographed lies within the managed forestry of Massey's Wood, and the ground cover and tree planting can change the character of the area considerably between seasons. Spring and early winter, when undergrowth is thinner, give the best chance of reading the topography clearly, even if the enclosure itself remains, as the record plainly states, without visible trace.