At first glance it presents as a ruin of modest dimensions, a single-storey vaulted structure roughly six and a quarter metres long and three and a half metres wide internally, with walls over a metre thick rising to about two metres, and a gable climbing to around four metres at its apex. Splayed loops pierce the east and west walls, a pointed doorway opens in the north wall, and an armorial stone sits above the entrance. It looks, in other words, like something with a purpose worth arguing about.

For decades, architectural surveys labelled it a chapel, first confidently in 1973, then more hesitantly in 1979, when one report hedged by calling it "apparently an early-17th century chapel, but so rustic as to give little indication as to its function, perhaps more likely a gate-lodge." Neither identification holds up well under scrutiny. The building is aligned north to south, which is atypical for a church; its fabric has clear affinities with tower-house rather than ecclesiastical architecture; and the "chapel" label appears to have originated in a misreading of an Ordnance Survey map, where the name "Old Chapel" was wrongly attached to this structure rather than to a separate building in the grounds of Longford House to the east. A far more persuasive clue to its actual function is a flue-hole broken through the apex of the vault, roughly two-thirds of the way south from the entrance, above which a chimney survives. The antiquarian W.G. Wood-Martin recorded in 1882 a local tradition that the building once served as a forge, and that hole in the vault fits neatly with that account. The punch-dressed stonework around the pointed doorway, large deliberate marks characteristic of later sixteenth and early seventeenth-century craft, provides the closest thing to a date. Local tradition holds that both the doorway and the armorial stone were transported here from an older castle to the west, though scholars consider the doorway itself most likely original to the building, while the armorial stone is probably the insertion. One curiosity in the south gable wall is a stone with a hole that superficially resembles a gun-loop but lacks the internal splaying that would confirm it; it appears instead to be a repurposed pintle stone, the kind used as a footing for a door hinge, set into the wall at a later date.