The stone is a beach-quarried piece of grit, waterworn and oval in cross-section, and it carries two quite different kinds of marks. Along its south-eastern edge runs a short ogham inscription, ogham being the early medieval script in which letters are represented by groups of notches and lines cut along a stone's edge or ridge. The inscription reads MARIANI, a Latin genitive form meaning "of Marianus". On the eastern face, cut into the same stone, is something harder to explain: a rectangular outline divided into four roughly equal parts, with the upper two quadrants each subdivided again into four smaller squares, all formed by incised lines. Nineteenth-century observers found it suggestive of a building plan, but no one has offered a convincing explanation of what it represents.

The stone attracted serious antiquarian attention from the 1840s onwards, and its early readers disagreed almost immediately. When the Reverend Charles Graves presented his findings to the Royal Irish Academy in June 1849, he argued that the name Marianus belonged to Christian times and suspected the inscription might be as late as the tenth or eleventh century. Writing in the 1850s, Nash pushed back, placing it no later than the fifth century and noting that Marianus was a well-known name in Irish ecclesiastical circles, suggesting the person commemorated was probably a cleric. A further complication arose from a drawing by Windele that circulated in the 1850s: Hitchcock complained that it made the stone look flat and square rather than oval, misplaced the inscription relative to the cross diagram, and omitted a small secondary cross near the base. That secondary cross, noted by Brash when he visited on 25 July 1868, is now buried beneath the surface. By 1945, when Macalister recorded the stone, it had been set deep into the earth against a modern grave, reducing its visible height from around 1.8 metres to 0.7 metres and making the inscription awkward to read. Linguistic analysis by McManus in 1991 placed it among the earliest ogham inscriptions in existence, dating it to the first half or early second half of the fifth century, on the basis that it shows no trace of vowel affection, a grammatical feature that developed later in Old Irish. The same name, in the same genitive form, appears on an ogham stone at Rathglass in County Carlow, which makes the pairing of the two monuments a small puzzle of its own.

The stone stands near the western side of the graveyard at Cinn Aird Thoir. Because so much of it is now below ground level, the ogham inscription along the south-eastern edge requires a close look, and the cross diagram on the eastern face, with its grid of subdivided squares, is best examined in raking light that brings the incised lines into relief.