A fosse, in this context, is simply a ditch dug as part of a defensive or boundary arrangement, the soil from which was typically thrown up to form the accompanying bank. What makes this remnant quietly odd is not what it is, but what it used to be, and what happened to most of it.

The first edition of the Ordnance Survey six-inch map recorded a rectangular enclosure on this spot, oriented roughly northeast to southwest at around forty metres, and northwest to southeast at around thirty metres. It sat on top of an esker ridge, one of those long, winding gravel and sand ridges left behind by glacial meltwater rivers during the last ice age, features that appear with some frequency across the Irish midlands and west and that were often favoured as dry, elevated ground for early settlement or activity. At Cloonascragh, the ridge itself was subsequently quarried away, taking the bulk of the enclosure with it. The three banks and two fosses that remain are essentially the portion that escaped removal, a surviving edge of something that was once coherent and purposeful, now readable only in outline against what the map once showed.