During excavations carried out ahead of its construction, archaeologists uncovered the partial outline of a subrectangular enclosure in the townland of Killydonoghoe, a modest but quietly telling find of the kind that bypasses tend to produce and motorists rarely think about as they drive overhead.

What survives, or rather what was recorded, consists of the northern and western sides of an enclosure measuring roughly 14.5 metres by 7 metres, defined not by a wall or bank but by a shallow flat-bottomed trench. Trenches of this kind could have held a timber palisade or some form of fencing, making the enclosure a defined domestic or agricultural space rather than a fortification in any grand sense. The trench itself was between 1.3 and 2 metres wide and no more than 0.4 metres deep at its deepest point. A possible entrance, around 2.8 metres across, was identified on the northern side, and the interior yielded evidence of pits and what may have been a hearth, suggesting that people lived or worked within this space. The excavation was reported by Sherlock in 2003. Without datable material mentioned in the record, the precise period to which the enclosure belongs remains uncertain, though enclosures of this general type are found across a wide span of Irish prehistory and the early medieval period.