The example at Mannin in County Mayo does something quietly different: it is classified as an inland promontory fort, meaning its builders found an equivalent configuration in the landscape without the help of the Atlantic. A spur of high ground, a river bend, a bog-bound tongue of land, something in the local topography gave them the same strategic logic, the same sense of a place almost surrounded by natural obstacles, and they worked with it.

Inland promontory forts are less frequently discussed than their coastal counterparts, but they are not uncommon in the west of Ireland, where the terrain offered plenty of naturally defensible projections of land. The basic principle is the same across all variations: reduce the perimeter you need to defend artificially by choosing a location where nature has already done much of the work. A single bank and ditch across the neck of the promontory could then enclose a reasonable area of ground with relatively modest effort. Who built the fort at Mannin, and precisely when, is not currently documented in the available record, but such structures in Ireland are generally associated with the later prehistoric or early medieval periods, when enclosed hilltop and promontory sites served communities as places of refuge, seasonal assembly, or high-status settlement.