A promontory fort, known in Irish as a dúnán or similar local variant, works on a straightforward principle: three sides are protected by sea cliffs or steep drops, and a single landward side is blocked by one or more earthen banks and ditches. The result is a defensible enclosure that required far less labour than an inland ringfort of comparable size. Along the Mayo coastline, where the Atlantic has carved the land into a succession of headlands and inlets, this type of monument appears with some regularity, each one a quiet mark of early medieval or Iron Age settlement.

An Bheithigh sits on the north Mayo coast, a stretch of coastline characterised by dramatic cliff scenery and relatively sparse modern development. Promontory forts in this region are generally associated with the Irish Iron Age or the early medieval period, roughly spanning the centuries around the turn of the first millennium, though precise dating of individual sites often remains uncertain without excavation. The choice of such locations was rarely accidental. Communities that depended on maritime resources, including fishing, coastal trade, and movement between islands and inlets, had particular reason to occupy and fortify headlands, which offered both a vantage point and a measure of natural protection.