Ogham is an early medieval script, typically carved as a series of notches and strokes along the edge or face of a stone, and used mainly to record personal names in an archaic form of Irish. What makes the Greenhill stone quietly complicated is that nobody has been able to agree on exactly what it says. The lower portion of the inscription has been worn by centuries of cattle rubbing against it and scored by whetstones used to sharpen blades, and that damage has sent scholars in different directions.

The inscription runs from about 45 centimetres above the ground almost to the full height of the stone. Richard Rolt Brash, working in 1879, read the opening word as TTGENU, a reading shaped by the damage already present at that point. Eoin MacNeill, writing in 1909, gave it as TRENO. R.A.S. Macalister, whose 1945 corpus of ogham stones remains a standard reference, offered TRENU MAQIMUCOI QRITTI, a formula that would translate roughly as "Trenu, son of the tribe of Crott", the MAQIMUCOI element being a genealogical marker common in ogham inscriptions. Damian McManus, reviewing the evidence in 1991, considered MacNeill's TRENO a possible reading, without settling the matter conclusively. In September 1985 the Office of Public Works installed a cattle grid around the stone to limit further damage, and a small excavation of the surrounding ground, a circular strip one metre wide, turned up nothing beyond north-south cultivation furrows and old ploughmarks. What the stone's immediate landscape was used for, and when, remains open.

The Greenhill stone is not isolated. Another ogham stone lies roughly 40 metres to the north in the next field, and the Burnfort ogham stone was found about 900 metres to the southeast, making this part of County Cork unusually dense with early inscribed monuments. The School of Celtic Studies at the Dublin Institute for Advanced Studies has since documented the stone as part of its Ogham in 3D project, producing high-resolution digital records of the surface to assist with readings that the naked eye, and centuries of livestock, have made difficult.