The sod-covered foundations are all that remain of a church that was once substantial enough to warrant careful surveying, yet had vanished from official mapping within a generation of first being recorded.

The Ordnance Survey's six-inch map of 1842 shows the building as a T-shaped structure, roughly twenty metres along its east-west axis and eight metres across, with a southern extension of approximately ten by five metres. The whole sat within a rectangular enclosure of around forty by thirty metres. By the time later editions of the same map series were produced, the church had been removed from the record entirely, suggesting the structure was levelled sometime in the mid to late nineteenth century. What the 1842 surveyors captured, then, is a near-final glimpse of a building already on its way out. When measured on the ground today, the buried foundations correspond closely to those early survey dimensions, running twenty-two metres east to west with the southern projection extending eleven metres, confirming that the cartographic outline was accurate and that the archaeology beneath the turf remains largely intact.

The T-shaped plan itself is worth pausing over. While the more familiar form for Irish rural churches is a simple rectangular nave, T-shaped plans occasionally indicate a later addition or a transept-like extension, sometimes associated with the accommodation of growing congregations or the insertion of a private burial aisle. At Clashganniv, the southern projection is substantial relative to the main body, though without excavation it is difficult to say more about its function or the period of the church's active use.