The sea defends three sides while the landward neck is blocked by earthwork and ditch, leaving a small enclosed area that could be held against attack or used for shelter and storage. The example at Dunbeacon, on the northern shore of Dunmanus Bay in west Cork, takes this logic to a modest extreme. The oval enclosure measures just 23.6 metres north to south and 13 metres east to west, barely nudging out into the bay. It is a tight, purposeful space rather than a grand fortification.

What defends the neck is nevertheless substantial: an earthen bank rising to 5.2 metres, fronted by an external fosse, a cut defensive ditch, descending to 2.65 metres deep. A causeway just 2 metres wide crosses both bank and fosse on the southern side, the only controlled point of entry. When the antiquarian T. J. Westropp visited in 1915 he recorded a dry-stone revetment facing the bank, a layer of carefully laid unmortared stone that would have kept the earthwork from slumping. That revetment is no longer visible. Westropp also noted a collapsed souterrain close to what he called the dangerous edge of the promontory. A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval settlement, used for storage or as a place of refuge. Its collapse, and its position near the cliff edge, suggest the interior has been losing ground to erosion for some time. The Ordnance Survey six-inch map of 1842 shows the interior as a circular area that has since reduced to less than half its recorded size, which gives some measure of how much has already gone into the bay.