The upper portion hangs on the south wall of St. Mary's church, while the lower portion has been repurposed as the church's altar at the east end. Together, the two pieces once formed a single commemorative structure roughly eight and a half feet tall and over seven feet wide, a substantial presence in what is a modest medieval building on a small island in Lough Derg.

The monument was erected by Lady Slaney Brien in memory of her son, Sir Terlagh Mac I Brien Ara, a baronet who died on 28 March 1626, and his wife, Elys Butler, daughter of Walter, Earl of Ormond, who had died the previous year on 10 February 1625. The O'Brien arms are carved into a circular depression at the centre of the upper section, surrounded by an inscription that, though badly worn, was transcribed and studied by the antiquarian R.A.S. Macalister in the years 1916 to 1917. He described the upper portion as a triangular pediment set within a moulded border and flanked by pinnacles, the kind of Renaissance-influenced funerary design that became fashionable among Gaelic and Old English nobility in early seventeenth-century Ireland. The altar section, for its part, carries a rougher carving of the crucifixion, flanked by stiff floral panels, the two halves of the monument now serving quite different purposes from one another and from what was originally intended.

The inscription closes with the Latin phrase "Memento Mori", a reminder of death conventional to monuments of this period, though here it carries a particular weight. The families commemorated, the O'Briens and the Butlers, were among the most prominent dynasties in Munster, and their intermarriage reflects the complex social world of the early seventeenth century, when old Gaelic titles and English earldoms still overlapped. That such a monument survives at all, even in pieces, on an island accessible only by boat, makes it a genuinely odd survival.