A promontory fort uses the sea itself as its primary barrier, leaving only the landward side to be secured with a bank, ditch, or wall. The result is a naturally fortified enclosure requiring far less labour than an inland ringfort of equivalent size, and the dramatic coastal setting is incidental rather than decorative.

Promontory forts of this type are found at intervals all along the Irish coastline, and most are thought to date broadly to the Iron Age, though some were constructed or reused during the early medieval period. The headland at Dunbur, just south of Wicklow town, would have offered both a defensible position and proximity to the sea lanes that carried trade and, occasionally, threat. Whether the occupants were a local ruling family, a seasonal garrison, or something else entirely is the kind of question the archaeology alone rarely settles. What survives at such sites is typically an earthen or stone rampart cutting across the neck of the headland, with the waves doing the rest of the work on three sides.