On a south-west-facing slope of a hillock in the undulating pasture around Longstone, Co. Tipperary, there is an enclosure that no Ordnance Survey map has ever recorded, that leaves no impression on the ground underfoot, and that reveals itself only when viewed from the air. It is, in the most literal sense, invisible to anyone standing in the field where it lies.

The enclosure was identified from an aerial photograph, reference GSI R442/1, which shows a roughly circular feature approximately eighteen metres in diameter. Aerial photography of this kind works by capturing the differential growth of grass or crops over buried features, subtle variations in colour or height that betray the outlines of walls, ditches, or banks long since levelled. The site never appeared on any edition of the six-inch Ordnance Survey maps that charted the Irish countryside in successive waves from the nineteenth century onward. A boundary running north-east to south-west, visible on the 1906 edition of those maps to the south-east of the enclosure, has since been removed, further erasing any surface trace of what was once here. What the enclosure originally was, and when it was in use, the surviving evidence does not say.