It is trapezoidal in shape, its side and end panels broken, and its surfaces carved in false relief with a writhing arrangement of interlaced animals and snakes, both short and long, in a style influenced by the Urnes tradition. Urnes is a Scandinavian decorative mode, named after a Norwegian stave church, in which elongated animals and serpentine forms loop and bite one another in dense, flowing patterns. Seeing it here, on a twelfth-century Irish tomb, is a reminder of how fluidly artistic influences moved across medieval Europe.

The sarcophagus has been dated to the 1120s or 1130s, and researchers have noted that its dimensions suggest it was originally designed to sit against the north wall of a different church on the Rock entirely, not the chapel where it now rests. It was moved to Cormac's Chapel in 1875, transferred from the cathedral for safekeeping. As for who it was made for, one persuasive suggestion is that it served as the tomb of Tadhg Mac Carthaig, who became king of Cashel after the death of his brother Cormac and died himself in 1124. Cormac Mac Carthaig was the patron behind the chapel that now bears his name, one of the finest pieces of Romanesque architecture in Ireland, so there is a certain poignancy in the idea that his brother's carved tomb has ended up sheltering within those same walls.