When the local historian T. O'Rorke described it in 1878, he counted three distinct lines of circumvallation encircling the summit, the outermost reaching some 54 metres in diameter. By the time the Ordnance Survey revised its mapping in 1913, the same feature was rendered by little more than a partial arc of hachures, the conventional marks used to indicate sloping ground. Where O'Rorke saw a fort of layered earthen rings, later surveyors found something harder to read.

Ringforts, known in Irish as raths, were typically enclosed farmsteads of the early medieval period, their banks and ditches marking off a domestic space from the world outside. The one at Larkhill is somewhat more complicated in its topography. The level interior measures roughly 26 by 20 metres, defined by a steep scarp that rises around two metres along most of its arc but merges with the natural slope of the knoll along the south, where the combined height reaches approximately 4.5 metres. A ramp-like extension projects eastward from the main enclosure for around 50 metres, tapering as it goes, with a raised oval projection at its north-eastern end flanked by natural depressions on either side. The marshy ground encircling the knoll's base adds a further layer of natural defence, making it difficult to say exactly where the built earthwork ends and the landscape begins. Beneath the southern half of the interior lies a souterrain, an underground passage or chamber of the kind often found associated with early medieval settlements, sometimes interpreted as storage space or as a refuge in times of danger.

The 1838 Ordnance Survey six-inch map shows the site clearly as an embanked circular area, with a trackway approaching from the south. That trackway, and most of the concentric detail O'Rorke admired, has since been absorbed back into the pasture that now covers the knoll.