A cashel is a stone-walled ringfort, the Irish equivalent of the more familiar earthen rath, and this one was already considered broken when the Ordnance Survey's correspondents passed through and recorded it in the nineteenth century. That description, "a broken Cahir on the top of Kilmacoo Hill", comes from the Ordnance Survey letters, a remarkable series of documents compiled during the great mapping project of the 1830s, in which local scholars and antiquarians wrote detailed accounts of the places they encountered. The word "cahir" is simply an anglicisation of the Irish "cathair", another term for a stone ringfort, and its use here gives the entry a pleasingly archaic texture.

The fact that the structure was already ruinous by the time it was formally noted tells its own story. Cashels of this kind were typically built during the early medieval period, serving as enclosed farmsteads or the residences of local lords, their thick stone walls defining a boundary between domestic life and the wider landscape. On a hilltop position like Kilmacoo, such an enclosure would have commanded considerable views and perhaps carried some defensive or territorial significance. By O'Flanagan's time, the stones had evidently been robbed out or tumbled, leaving only the broken outline that his letters describe. That outline is what remains today, a site more legible to the imagination than to the eye.