What survives at Glenidan is not a mound at all but a hollow, a circular concave depression roughly five and a half metres across, ringed by a well-preserved earthen bank. The bank itself is substantial enough, stretching nearly four metres wide and rising to a maximum external height of about one and a half metres. It is the shape of the thing that puzzles: rather than projecting upward from the ground as a mound barrow, a type of prehistoric funerary monument constructed as a raised earthen heap over a burial, it dips inward at its centre.

The most plausible explanation is that this was originally a bowl-barrow, one of the rounded, dome-shaped burial mounds found across Ireland and Britain from the Bronze Age onward, and that someone at some point quarried material from its centre, digging out the core of the mound and casting the excavated soil outward. That displaced earth would account for the encircling bank that now defines the monument. Whether this quarrying was deliberate robbing of the mound, casual extraction of useful soil or gravel, or something else entirely is not recorded. The scrub vegetation that has colonised the monument makes detailed examination difficult, and the original profile remains uncertain. A ringfort, the circular earthwork enclosure common across early medieval Ireland, sits about 75 metres to the north-east, suggesting this small corner of Glenidan has been in more or less continuous use, or at least occupation, across a considerable span of time.