It is only about 0.4 metres high and two metres wide, oriented roughly west-northwest to east-southeast, yet its shape is what catches the attention of anyone looking closely: a horseshoe, roughly 17 metres north to south and 12 metres east to west, defined by a low encircling bank with the interior ground sloping away to the south.

What this feature originally was remains genuinely uncertain. The most likely interpretation is that it began as a mound, the kind of low earthwork that appears throughout Ireland in various prehistoric and early medieval contexts, sometimes used for burial, sometimes for assembly or boundary marking. At some point, it appears to have been partially quarried into, which would account for both the horseshoe plan and the eroded southern interior. Quarrying into existing mounds was not uncommon in earlier centuries, when the material was useful for drainage or levelling, and the process often left behind exactly this kind of scooped, open-ended profile. A separate enclosure, a roughly bounded area defined by an earthen bank or ditch, lies about 180 metres to the southeast, suggesting this part of Knockfobole held some significance across a wider area rather than as an isolated anomaly.