The label "Old Chapel" first appeared on the Ordnance Survey map edition of 1913, yet the same building appears on the earlier 1837 edition without any such designation. What it actually was, nobody can say with certainty.

The building is a single-storey, gabled rectangle, mortared stone, aligned roughly north-north-west to south-south-east, with walls between 0.45 and 0.55 metres thick. An arched doorway in the north gable provides the entrance, and the east wall carries a second arched opening. A narrow ope, a small aperture just 0.44 metres high, sits in the upper portion of the south gable. The upper section of the structure was likely rebuilt and reslated sometime in the twentieth century, which complicates any attempt to read its original form. The working interpretation is that this was an ornamental building, the kind of deliberate folly or garden feature that was fashionable on eighteenth-century demesnes, designed less for use than for atmosphere, and possibly constructed to display stone carvings that are set into its walls. That a pre-1700 date cannot be ruled out only deepens the uncertainty. Wood-Martin noted the structure in 1882, and the grounds of Longford House were surveyed again in the 1970s, but the question of function has never been resolved. A building recorded as a chapel that almost certainly never held a congregation, on an estate whose own history has largely slipped from common knowledge, sits quietly collecting the wrong name.