That last detail is what makes Dundeady quietly unsettling: the possibility that part of the fortified landmass has simply been consumed by the sea, leaving the surviving defences as the edge of something that used to be larger.

A promontory fort is one of the oldest forms of coastal defence in Ireland, typically created by throwing up an earthen bank or stone wall across the narrow landward approach to a headland, letting the sea cliffs do the remaining work on the other sides. At Dundeady, the visible defences are medieval in date, but the antiquarian Thomas Johnson Westropp, writing in 1914, thought they might be sitting on top of something older. The ground dips to the north of the curtain wall in a way that looks like a fosse, the ditch commonly cut in front of an earthwork to increase its defensive height, though the feature is more likely a natural depression than a deliberate excavation. Westropp also raised a more arresting possibility: that Dundeady was once one of several long, narrow headlands running out near the lighthouse, and that erosion has since swept most of them away. If he was right, the fort as it originally stood occupied a geography that no longer exists.