This defensive site, marked on the 2nd and 3rd editions of the Ordnance Survey 6-inch maps, occupies a south-facing promontory that juts into the water. Natural defences provided by a 4-metre drop on the eastern side are complemented by human-made fortifications on the western approach; a fosse measuring 5 to 6 metres deep, followed by a defensive bank and an outer fosse.

Archaeological evidence suggests these defensive features once extended along the northern, landward side of the promontory as well, effectively cutting off access from the mainland. Unfortunately, modern road construction has damaged or destroyed much of this northern section, making it difficult to fully understand the original extent of the fortifications. The surviving features, however, still clearly demonstrate the strategic thinking behind the fort’s placement and construction.

The site represents one of many coastal promontory forts found throughout Ireland, where communities took advantage of natural topography to create easily defensible positions. These forts, which date from various periods in Irish prehistory and early history, typically combined natural cliffs or steep slopes with artificial banks and ditches to control access to a protected area. At Doonalin, the combination of its commanding position over Mill Bay and its substantial defensive works suggest it once played an important role in controlling or monitoring maritime activity in this part of Donegal.