A promontory fort of this type works on a straightforward principle: the sea does most of the defensive work on three sides, and a constructed wall seals off the landward approach, turning a finger of clifftop into an enclosed space with natural protection on nearly every side. At Ballyheer, that wall curves slightly, runs six to eight feet thick, and still stands three courses high on the outside face. The interior, measuring roughly 78 feet by 38 feet, is described as featureless, which itself says something about how thoroughly the site has been reduced.

The scholar T. J. Westropp recorded the fort in 1922, noting its basic dimensions and the presence of an external fosse, a defensive ditch, with a stone wall running along its base. From the air, that outer fosse remains legible, even as the fort itself has suffered considerably on the ground. A nearby ringfort, a roughly circular enclosed settlement of the kind found across Ireland, was also cleared away, and much of both structures was apparently broken up and used to build a bank along the cliff edge. That kind of secondary use is common enough in the archaeological record, where older stonework gets quietly absorbed into field boundaries or drainage works, but it means that what survives here is a much-diminished version of what Westropp observed, and the site has not been formally visited or assessed since his time.