The example at Frenchbrook in County Mayo makes that boundary work harder than most. Positioned at the northern edge of a small limestone ridge, the mound sits precisely at the break of slope, where the ground drops away sharply to the north and north-east into a natural hollow. The builders compensated for this unevenness with an asymmetry that is still visible: the mound stands only 1.45 metres high on its western side but rises to 2.4 metres on the east, and on the north-east to south-east arc the slope is noticeably steeper, engineered to maintain a consistent profile against a hillside that refused to cooperate.

The mound itself is a flat-topped circular form, roughly 13 metres in overall diameter with a level upper surface of 7.3 metres across, constructed from a mixture of earth and stone. Around it, the fosse, about 1.8 metres wide, survives as a very shallow depression on the western and northern sides, flattening into something closer to a terrace on the north-east to south-east stretch. Beyond the fosse, a low external bank, between 1.2 and 1.4 metres wide and only about 0.25 metres high, can be traced running from the north-west around to the north-north-west before it fades out. Later field walls cutting across the southern and south-western edges have clipped both the bank and the fosse, giving the monument a slightly truncated appearance on that side. Ten metres to the south-south-east, on the same hilltop, sits a cashel, a type of stone-walled early medieval enclosure typically used as a farmstead or small settlement, making this particular ridge an unusually layered piece of ground, with the prehistoric burial and the early medieval habitation placed in close proximity on the same spur of undulating limestone.