A portal tomb, sometimes called a cromlech, is one of the oldest monument types in Ireland, typically consisting of two or more upright stones supporting a large capstone, and built as a megalithic burial chamber in the Neolithic period. What makes the Lennan example quietly peculiar is not just its age but a carved inscription on one of its damaged side-stones, one that no one has satisfactorily explained.

The monument appears in the historical record at least as far back as 1835, when the scholar John O'Donovan noted a scribbing at the edge of a fissure on the western side-stone. A second record was made by Graves in 1848, later reproduced by Ferguson in 1873. Both observers noted the carving, but what they found resisted easy classification. It does not appear to be ogham, the early medieval Irish script that runs in notched lines along stone edges, nor does it resemble any other known Irish inscription. If anything, it has been compared to runes, though no firm identification has ever been established. The roofstone had already been dislodged and the side-stone damaged by 1847, so the carving may have suffered further since O'Donovan first saw it, and its present condition is not known. The monument was built into the outer edge of a field bank at the south-west corner of a field; that bank had been removed by 1983, leaving the tomb more exposed. By that point it was recorded as a substantial structure, with a roofstone measuring 3.4 metres by 2.6 metres and 0.7 metres thick, resting on two side-stones. The 1907 edition of the Ordnance Survey six-inch map marks it in gothic lettering as a Cromlech, though it does not appear on earlier editions.