It is, in fact, a side panel from a chest tomb, a box-shaped raised tomb that was a common form of elite burial in late medieval Ireland, and what it carries in carved relief is quietly remarkable: a full pictorial inventory of the Arma Christi, the instruments of Christ's Passion, arranged across four sections and still legible after several centuries of damage and repair.

The slab measures 1.81 metres long and 0.61 metres high, divided originally by fluted columns, though the leftmost column is now lost. Reading from right to left, the sections move through the narrative of the Passion in a kind of compressed visual shorthand that medieval viewers would have recognised immediately. The first section shows a scourging pillar wound with ropes, a scourge, a seamless robe, what appears to be a towel, and a pot that once likely supported a carved cockerel, the bird whose crowing marked Peter's denial. The cockerel itself is gone. The next section presents a ladder flanked by a spear and a cup on a pole, instruments associated with the Crucifixion. The third section is perhaps the most densely packed: a cross encircled by a crown of thorns, accompanied by pincers, a hammer, and three dice, the last a reference to the casting of lots for Christ's garments. The fourth and final section was devoted to the five wounds of the crucified Christ, though only the carved feet now survive, the upper portions of that corner having broken away and been re-attached at some point, imperfectly.

This kind of Passion iconography on tomb panels was not unusual in late medieval Irish ecclesiastical settings, but few survive so legibly or in such concentrated detail. The upper corners of the slab are damaged, and the missing column means the divisions between sections are now partly implied rather than stated, yet the programme of imagery holds together. St Mary's in Callan is a substantial medieval church, and the panel sits at the western end of the north aisle of the nave, where the light is low and the stonework around it largely unadorned, which makes the carved relief, worn as it is, stand out rather more than might be expected.