It is sphinx-like in form, the face flattened so that the ears sit at either end of the forehead rather than at the sides of the skull, as though the medieval carver wanted to compress the full three-dimensional head into a single plane. From each ear, a creature attacks. On one side, a quadruped prances downward and twists its head back at a right angle to bite. On the other, something stranger: a fish-bodied beast with a dragon's head, fierce teeth, and fins bristling along its back, its tail curled beneath the chin of the human face. Below the face, diagonal lines radiate outward from the chest, which may represent ribs, or a body in the process of decomposition.

The scholar Peter Harbison, writing in 1973, read the scene as possibly depicting either the temptation of man by the devil or the torments of hell, with the fantastical animals standing in for the devils that perpetually devour the damned. Romanesque in inspiration, the head was probably carved sometime in the twelfth century, and comparable motifs appear at Glendalough in County Wicklow and at Dysert O'Dea in County Clare. The church itself has a layered history. By the time Samuel Lewis was writing his Topographical Dictionary in 1837, St. Stephen's Church of Ireland building incorporated the nave of an older medieval structure, its belfry thick with ivy. Lewis noted a mural monument to the Meridiths of Dicksgrove inside, and recorded that the Ecclesiastical Commissioners had granted £290 for repairs. The west tower of the ruin is now understood to represent what survives of the medieval parish church of Castleisland. The 1841 Ordnance Survey six-inch map shows two churches occupying the northern part of the graveyard; later revisions show only one, St. Stephen's, remaining. The sculptured head, which Lewis tentatively identified as a possible portrait of St. Nicholas, the supposed patron saint of the earlier church, had already been built into the exterior south wall by that point, repurposed and embedded in later masonry, its original context lost.