Known in Irish as the Leac Leó, meaning roughly "the flag" or "flat stone of Leo", it is a small monument with an unusually restless history, appearing and disappearing across maps and records over more than a century before vanishing entirely.

The stone was recorded on the 1839 Ordnance Survey six-inch map as "St. Leo's Flag", positioned about sixteen metres to the south-east of St Leo's Chapel on the island. The Ordnance Survey Letters of the same period, compiled by Rev. M. O'Flanagan from fieldwork carried out in 1838, called it a "stone Cross called Leac Leó". By the time the map was resurveyed in 1898, its name had quietly shifted to "St. Leo's Flag (Site of)", the parenthetical suggesting some uncertainty about whether it still stood, and its recorded position had moved to the east side of a nearby roadway, about thirty-five metres from the chapel. Then, in 1905, a published guide to the western islands reported that the cross had been relocated again, this time set into the east gable of the chapel itself. At that point it was described as bearing a chalice carved on one face and, on the other, a human figure with arms outstretched, believed to represent a bishop. It is a cross-slab, a category of early medieval carved stone relatively common in the west of Ireland, though examples with figurative carving of this kind are comparatively rare. When a fieldworker checked both its earlier recorded locations in August 1984, neither produced the stone. The chapel gable, presumably, yielded nothing either.

Inishshark was permanently evacuated in 1960, when its remaining inhabitants were resettled on the mainland, so there is no community left to hold any local memory of where the Leac Leó ended up. The island is accessible only by private arrangement, and the chapel itself is a roofless ruin. Whether the stone is buried, broken, or simply overlooked somewhere in the grass is unknown.