The site at Knockderk is a ring-barrow, a type of prehistoric funerary monument consisting of a low central mound surrounded by a circular ditch or bank, and it left so little impression on the landscape that it was never recorded on the Ordnance Survey's historic maps, despite those surveys having documented Irish townlands in considerable detail since the nineteenth century.

The monument only entered the archaeological record in 1986, when it was identified during the Bruff aerial photographic survey, recorded under reference Bruff 94 and AP 4/3616. Aerial survey has been one of the most effective tools for discovering sites of this kind, because ploughing, drainage, and centuries of agricultural activity can reduce a barrow to nothing visible on the surface while the buried ditches and banks still influence soil moisture and crop growth differently from the surrounding ground. That differential shows up as a cropmark, and a faint circular one remains visible on Ordnance Survey orthophotography taken between 2005 and 2012, and again on a Google Earth image dated 18 November 2018. The site sits roughly 60 metres east of the townland boundary with Ballynagreanagh, and a ringfort, a type of enclosed farmstead common in early medieval Ireland, lies about 170 metres to the north-west, suggesting the broader area has seen human activity across several periods.

Because the monument survives only as a cropmark, there is little to observe during a ground visit without specialist equipment or prior knowledge of precisely where to look. The wet pasture setting means the ground can be soft underfoot, particularly in autumn and winter. Anyone with a serious research interest would do better to begin with the aerial images compiled by Martin Fitzpatrick and uploaded to the record in April 2021, which give the clearest picture of what survives. For the general visitor, the value here is less in what can be seen and more in what the site represents: a burial place old enough, and subtle enough, to have gone entirely unrecorded until a light aircraft passed over at the right angle on a dry summer's day.