A rath, as this type of monument is commonly called, is a roughly circular enclosure defined by an earthen bank and external ditch, used as a defended farmstead during the early medieval period in Ireland. At Grenagh, the enclosure measured approximately 35 metres in diameter, a modest but typical size, and was recorded in enough detail on the 1842 Ordnance Survey six-inch map to show its full circular outline with hachuring, the fine parallel lines cartographers used to indicate a raised bank or earthwork.

What makes the site's story legible is the way successive maps trace its gradual disappearance. By the time the 1842 survey was made, a field fence had already been driven east to west straight across the southern half of the interior, a practical agricultural decision that effectively bisected the monument. Later maps from 1904 and 1938 show the bank and its external fosse, the shallow defensive ditch that would have run around the outside of the enclosure, still surviving to the north of that fence line, but already the southern portion was gone. By the time of more recent observation, even the northern remnant had been levelled, leaving only a faint low rise in the ground as evidence of what had stood there. South of the old field boundary, no surface trace remains at all.