Running northwest to southeast across the narrowest point of Inis Mhic Aoibhleáin, this field boundary is not a ruin in any dramatic sense, but it carries the logic of a working landscape, a decision made in earth and stone about where one part of the island ends and another begins.

The structure itself is a double-bank arrangement with a ditch between. The main bank averages 0.8 metres in height and 2 metres in width, with a ditch roughly 1 metre wide cut into its northern side, the base of which sits 1.2 metres below the crest of the bank. A second, lower bank, reaching only 0.4 metres in height, runs along the northern edge of that ditch. What makes the boundary slightly more than a simple earthwork is the partial stone revetment on the northern face of the main bank, a facing of roughly coursed stones used to stabilise and reinforce an earthen structure, present only along the section closest to the eastern shore of the island. Why the revetment was confined to that stretch alone is not recorded, but it suggests either a specific vulnerability on that side, or that the work was never fully completed.