At roughly 15.8 metres in diameter and standing about 2 metres high, it reads at first glance as a reasonably intact prehistoric monument, the kind of stone mound that dots Irish hilltops from Donegal to Kerry. Look more closely, however, and the picture becomes more complicated. A cairn is a burial or ceremonial mound built from piled stone, and this one belongs to a specific type: a kerbed cairn, meaning it was originally edged with a ring of larger upright or recumbent stones to hold the structure in place. Some of those kerbstones are still visible to the north-east and south-west, but the western and southern sections have collapsed considerably, and a good portion of what visitors see today is the result of reconstruction carried out in the early twentieth century rather than the work of prehistoric hands.

The site was noted by scholars in the late nineteenth and early twentieth centuries. McCready recorded it in 1893, and Westropp returned to it in 1922, both documenting its condition and dimensions at a time when antiquarian interest in such monuments was growing alongside, unfortunately, a certain enthusiasm for tidying them up. The rebuilding that took place in this period was not unusual for the era; well-intentioned conservation efforts frequently involved restacking fallen material in ways that may have altered the original profile or obscured archaeological detail. What remains is therefore a layered object: a prehistoric monument whose present form has been shaped as much by Edwardian-era intervention as by its original builders.

Shelmartin is accessible on foot from the Sutton area, and the summit offers a clear sense of why such a prominent hilltop was chosen for a monument of this kind. Visibility from the cairn extends across Dublin Bay and towards the mountains to the south, which is typical of the siting logic behind many Irish hilltop cairns. The kerbstones worth seeking out are on the north-east and south-west edges; they are low and unobtrusive, easy to miss if you are not looking deliberately. The collapsed sections to the west and south give a rough sense of the original mass of the structure before it was partially reworked. There is no formal signage or managed access, so the cairn sits quietly in the landscape, waiting to be noticed by those who already know to look for it.