It measures roughly 22 metres long, 30 metres wide, and rises to about 2 metres in height. What it actually represents is a fulacht fia, one of the most common yet persistently puzzling monument types in the Irish archaeological landscape. These are the remains of ancient burnt mounds, accumulations of fire-cracked stone and charcoal that built up over repeated use, typically dating to the Bronze Age. The prevailing theory holds that they functioned as cooking sites, where stones were heated in a fire and then dropped into a water-filled trough to bring it to boiling point, though uses ranging from textile processing to brewing have also been proposed.

The horseshoe or kidney shape of the mound at Killeagh is characteristic of the type. It forms around the central trough where water was heated, the discarded burnt stone piling up on three sides over many cycles of use. The site sits on the eastern bank of a stream, which would have been essential to its function, providing a ready water supply for whatever process was being carried out there. A depression visible on the south-eastern side of the mound is the result of later quarrying, suggesting the material was at some point recognised as useful fill or stone, even if its prehistoric origins were not fully understood by whoever took from it.