That small deflection is easy to miss, but it speaks to something older than the wall itself. The mound in question sits at the southern end of a drumlin ridge, a low elongated hill shaped by glacial drift, on the crest of its south-facing slope. It is roughly oval, measuring about fifteen and a half metres east to west and eleven metres north to south, and rises to a modest height of just under two metres. Covered now in grass and trees, it reads at first glance as a natural feature of the landscape, which is perhaps why the wall builder chose to skirt around it rather than cut straight through.

The mound is a cairn, a term broadly used in Irish archaeology for a heap of stones, often prehistoric in origin and sometimes associated with burial. The 1835 edition of the Ordnance Survey six-inch map recorded it as a small circular enclosure, suggesting it was already visible and distinct enough to note, though its purpose was not yet labelled. It was not until the 1945 revision of that map that the feature was specifically identified as a 'Carn'. The name of the townland itself, Aghadrumcarn, carries the mound within it: the Irish word carn is embedded in the place name, hinting that local memory of the feature predates the cartographers by some considerable distance. The slight truncation of the mound's southern edge by the field wall suggests that even as the landscape was divided and farmed in more recent centuries, the cairn was recognised as something worth respecting, if not necessarily understood.