The carvings are small, the largest motif spanning roughly 21 centimetres, and they face northeast toward the valley. The boulder itself is low and fractured, barely 35 centimetres at its highest point, sitting in mountain heath with the mountains of Kerry rising to the south, west, and northeast. It is easy to pass by without noticing it.

Cup-and-ring marks are among the most widespread yet least understood forms of prehistoric rock art in Ireland and Britain. They consist of a central depression, the cupmark, surrounded by one or more concentric carved rings, and their purpose remains a matter of genuine scholarly debate. The Letter example is notable for its composition. Two cup-and-three-ring motifs are carved so that they share their outer rings, forming a conjoined, symmetrical pair. To the east of these, a third motif of a different type abuts them: a so-called rosette, in which a central cupmark roughly 2.5 centimetres across is encircled not by continuous rings but by seven smaller cupmarks arranged around it. Together, these three elements form a trefoil arrangement, a design that feels deliberate and considered rather than accumulated piecemeal. A separate, simpler cup-and-ring motif sits roughly 67 centimetres to the northwest, its ring incomplete, probably due to weathering. Four small pickmarks occupy a slightly raised ledge defined by a natural step in the rock surface, sitting just to the northeast of the main group.

The carving is very finely executed, but it has suffered. Repeated freeze and thaw cycles have fractured the sandstone surface and eroded parts of the northern conjoined motif, as well as possibly other features that are no longer legible. The watercourse draining from Coomnacronia Lake runs just a metre to the east, and the boulder sits where a smaller stream converges with it. That particular placement, beside moving water on an open slope with a long view down a river valley, is a pattern seen at other rock art sites in Ireland, though what it meant to the people who made these marks remains unknown.