It serves as the innermost lintel of a clochan, one of the dry-stone beehive huts characteristic of early medieval Irish monasticism, yet the slab itself belongs to an older or at least different purpose. It measures 2.1 metres long, 0.34 metres wide, and 0.27 metres thick, substantial enough that builders plainly chose it with intention, and its lower face carries an incised linear cross with short crosslets at the ends of each limb. That carving now faces downward into the doorway of Cell B, sheltered from the elements, which is perhaps the only reason it survives in the condition it does.

The slab is made of garnet mica-schist, a metamorphic rock that would have required some effort to source and work, and it is cracked at one end. It forms part of the masonry on both sides of the doorway, meaning it was not simply dropped into place as a lintel but was built around, integrated into the structure of the cell. Researchers have suggested the stone may originally have stood upright, functioning as a termon marker, that is, a boundary stone defining the sacred or legally protected precinct of a monastic site. High Island, known in Irish as Ard Oileán, was the location of an early Christian monastery associated with Saint Féchín, and the island's enclosure wall, church, and associated clochans represent a layered complex of monastic building that continued to be modified and reused over generations. The repurposing of a possibly freestanding boundary marker as a building stone, even a structural lintel, speaks to the pragmatic, cumulative way in which these communities worked with the materials available to them on a small, exposed Atlantic island.

Access to High Island is possible only by boat from the Connemara coast, and landings depend entirely on sea conditions, which can be unpredictable even in summer. The cross carved on the underside of the lintel is not immediately visible from the outside and requires close attention to the stonework around the doorway of the cell to appreciate its context within the surviving fabric of the site.