When archaeologist M. J. O'Kelly dug here in 1963, the interior yielded nothing at all. No hearths, no pits, no artefacts, no signs of habitation. In the language of field archaeology, the interior was sterile, which raises an obvious question about why someone went to considerable trouble to build the thing in the first place.

A ringfort, sometimes called a rath, is an enclosed farmstead of the early medieval period, typically a circular area surrounded by one or more earthen banks and ditches. At Garryduff, the builders were not modest in their effort. The enclosing bank rises to around eleven and a half feet, and the fosse, a rock-cut ditch running around the outside, was dug to a depth of eight feet. The enclosed area measures roughly ninety-five feet north to south and a hundred and fifteen feet east to west. O'Kelly's excavation also revealed that the bank had been faced in stone on its inner side, with additional stonework around the entrance, which sits to the east and spans sixteen feet across a causeway over the fosse. At some point a timber palisade partially closed that entrance, adding another layer of deliberate construction to a site that, as far as the excavation could determine, nobody ever seemed to live in. Its neighbour, Garryduff I, which O'Kelly excavated at the same time, is a separate enclosure and a distinct site in its own right. Whether the two were built together, or sequentially, or served related functions, the ground has not given a clear answer.