Just twenty-five metres north of the parish church, outside the graveyard walls entirely, lies a souterrain, one of those stone-lined underground passages or chambers built in early medieval Ireland, most commonly associated with storage or refuge. Its presence here, just beyond the boundary, is quietly anomalous, a reminder that what looks like a churchyard today was almost certainly a more complex site long before any Christian community claimed it.

The graveyard itself is oval in plan, measuring roughly fifty metres east to west and forty-four metres north to south, and is defined by a stone-revetted earthen bank, meaning the inner face of the enclosing bank is faced with stone to hold the earthwork in shape. Oval enclosures of this kind are generally considered an early medieval form, predating the rectangular plots that became standard in later centuries. The church of Kilnahue sits within this enclosure, and just to its north lies a cross-base, now displaced and lying loose on the ground, the socket stone that would once have supported an upright stone cross. The site occupies the lower part of a north-facing slope, close to the source of a small stream that runs south-east to north-west before joining the River Lask about seven hundred and fifty metres to the north-west, a modest but telling piece of geography, since early ecclesiastical sites were frequently founded near water sources.