The slab measures just 67 centimetres tall and 47 centimetres wide, yet its carved surface is carefully composed: a Greek cross rendered in four incised lines sits within a two-line square, with two-line circles filling the diagonal spaces between the arms of the cross, the angles of the cross hollowed out and every joint neatly mitred. When the antiquarian R.A.S. Macalister examined the stone in the early twentieth century and published his observations in 1916 to 1917, he dated it to the eighth century and described the geometry of the carving in some detail. What he apparently missed was the inscription running in a single horizontal line across the two upper quadrants of the cross, written in half-uncial script, the rounded early medieval letterform common in Irish ecclesiastical contexts.

The text, as later scholars read it, runs MUIR[-]A[I]TH, with one letter uncertain and another partially damaged. Okasha and Forsyth, writing in 2001, noted that if the penultimate letter is a C rather than a T, the whole word could be read as a form of Muiredach, a personal name well attested in early Irish sources. Whether it commemorates a monk, an abbot, or a lay patron buried somewhere on this island monastery, nobody can say with certainty. The stone had already had a complicated history before Macalister ever saw it. When it was first recorded in 1880, it was lying in the graveyard, no longer in its original position. By the time of Macalister's visit it had been moved into the church, where it remains today, fixed to the wall roughly 1.6 metres from the east end of the nave. Inishcaltra, also known as Holy Island, was an important monastic site associated with St. Caimin, and the presence of a carefully carved, inscribed stone of this quality fits the pattern of a community that took the marking of significant individuals and moments seriously, even if the precise meaning of this particular marker has slipped just out of reach.