The slab, measuring roughly 91 centimetres tall and 73 centimetres wide, was originally a grave marker, and at some point during the island's long monastic history it was lifted from its original position and built into the foundation stones of the east gable of the church on High Island. That it survived at all, let alone with its carving intact, is quietly remarkable.

The slab is cut from blue-grey mica-schist and carved on its east face with an expansional cross in relief, a type in which the arms of the cross broaden as they extend outward, filling the available space with deliberate geometry. The D-shaped terminals reach almost to the edges of the stone, framed by a bold raised band that also encircles the central roundel. The terminals themselves are filled with fretwork, a form of interlaced geometric ornament, and four raised bosses sit in the quadrants where the arms of the cross meet. In the upper angles, pairs of bosses are connected by C-scrolls or peltae, the pelta being a curved, shield-like decorative motif borrowed ultimately from classical art and absorbed into early medieval Insular carving. The grave it once marked, designated Grave 4 in the archaeological record, was a dual-phased burial: a first interment that pre-dates the church itself, and a later restructuring carried out when the church was built. Skeletal remains recovered from the grave have been dated to the late tenth or early eleventh century, placing the burial at a moment when the island's monastic community was still active.

High Island, known in Irish as Ardoileán, lies several kilometres off the Galway coast and is accessible only by arrangement, conditions permitting. The church and graveyard occupy a dramatic and exposed site, and the cross-slab, now embedded in the gable wall rather than standing upright, requires a close look to appreciate the quality and completeness of its carved decoration.