It is the grave cover of a chest tomb, a type of raised box-shaped monument common in post-medieval Ireland, and it bears a Latin inscription that was not recorded until a summer afternoon in 1868, when a writer and a few companions copied out what they could make of it. The date carved into the stone was already effaced by then, and so were portions of the lettering, leaving the monument in a state of deliberate incompleteness.

The inscription, as transcribed in 1871 by Lenihan, reads in part: "EGO. TEREN. BRIEN. HOC. IN. MEO. ET. IN. MEE UXORIS. MORIN. CARYL. NOIE. FIERI. FECI." Roughly rendered, Terence O'Brien of Arra declares that he caused this monument to be made in his own name and in the name of his wife, Morine or Matilda Carroll. The tomb belongs to the O'Brien family of Arra, a branch of the Uí Briain Aradh, and it sits within the grounds of a church that itself carries considerable history. The ruins, which stand on low-lying ground with the eastern shoreline of Lough Derg only seventy metres to the west, were described in the mid-twentieth century as being of post-Reformation construction, though the site served as a burial place for generations of Clann Brian Ruadh and Brian Ban chieftains. Among those commemorated here is Bishop Moriertagh Mac Uí Briain, educated at Oxford and the first Protestant Bishop of Killaloe, a figure whose biography speaks directly to the upheavals of the Reformation in Ireland. His son, Sir Tirlagh Mac Uí Briain, was buried elsewhere, at Iniscaltra on the Shannon, reportedly in a friar's habit, a detail that contemporary State Papers recorded with evident disapproval. The church at Castletown also stood within a cluster of medieval structures: a castle ninety metres to the north, a medieval mill to the east documented on the 1654 to 1655 Down Survey map, and a second castle site to the south-south-east.

The tomb cover now rests recumbent against the north wall, separated from whatever chest structure it once capped. A drawing of it was made in 1868, and a three-dimensional digital model has since been produced, offering a way to examine the carved surface in detail that the weathered stone itself no longer permits.