Built into the external eastern wall of an early medieval church, this carved slab of garnet mica-schist served double duty: grave marker on one side, building material on the other. The concealed western face, pressed against the stonework, carried its decoration in darkness for an unknown span of time before scholars recorded what was there.

The slab, which measures 0.94 metres high and 0.39 metres wide, is one of three decorated cross-slabs associated with Grave 3 in the south-eastern end of the island's graveyard. Cross-slabs are early medieval stone markers incised with a cross, often found at monastic sites across Ireland, and this example is notably short-armed and cruciform in outline. The visible eastern face bears three incised circles arranged as a double roundel on the cross-head. The western face, recorded by Fisher in 2014, carries a more elaborate composition: a cross with expanded arms and sunken armpits, at the centre of which a double roundel encloses a linear Greek cross laid over an outline Greek cross with flaring arms, two crosses occupying the same carved space. The slab may predate the church itself, possibly belonging to an earlier phase of burial activity on the site before any structure was built around or against it.

The original is no longer on High Island. It passed into the care of the Office of Public Works and is now held at their depot in Athenry, Co. Galway. A replica stands in its place at the grave, so visitors to the island's monastic remains will see a copy rather than the carved garnet mica-schist itself, though the form and decoration have been faithfully reproduced.