The example at Doontrusk in County Mayo takes a different approach entirely, applying the same logic to an inland setting, where a natural spur or ridge of land, rather than a cliff above the Atlantic, would have provided the elevated ground and restricted approach that made this form of enclosure so effective. The very place-name offers a clue: "Dún" is the Irish word for a fort or stronghold, suggesting that the defensive character of this place was legible enough to become embedded in local memory long after the structure itself fell silent.

Inland promontory forts are considerably rarer than their coastal counterparts, which makes Doontrusk quietly anomalous in the broader landscape of Irish prehistoric and early medieval fortification. The basic principle, cutting off a naturally defensible tongue of land with one or more earthen banks and ditches across its narrowest point, is well understood, but choosing an inland spur rather than a sea cliff implies a deliberate reading of local topography, and perhaps a different set of threats or social circumstances than those faced by communities perched above the ocean. County Mayo has no shortage of defensive monuments from the Iron Age and early medieval periods, but an inland example of this type sits somewhat outside the expected pattern, which is part of what makes it worth attention.