Among its quieter curiosities is a prehistoric cairn, a mound of heaped stones built by people whose intentions and identities remain unrecorded, sitting in a valley fold between two hills as though it had simply grown there.

The cairn occupies Thorn Chase valley, positioned between Heath Hill to the south and Tinians Hill to the north, and it looks out eastward over a coastal feature known as Seal Hole. To the west, the view is largely closed off by the surrounding terrain. The mound itself is grass-covered, though cairn material, the loose stony fabric beneath the turf, remains visible on the eastern side. Recorded and compiled by archaeologist Geraldine Stout, the structure measures fourteen metres in diameter and stands just under a metre high, at 0.95 metres. There are no kerbstones evident, which distinguishes it from many better-documented cairns elsewhere in Ireland where a ring of upright or recumbent stones defines the outer edge. Kerbstones typically serve both a structural and a ceremonial purpose, holding the cairn material in place while also marking a boundary between the monument and the surrounding ground. Their absence here leaves the mound with an informal, almost incidental quality. One further oddity of its form is that the northern face is noticeably steeper than the southern, suggesting either differential erosion or something in the original construction.

Lambay is privately owned and visits require prior arrangement; the island is not open to casual day-trippers. Those who do gain access will find the cairn in the interior valley rather than on the exposed coastal margins, so the approach involves some walking across uneven ground. The eastward aspect of the site means morning light falls across the area where the cairn material is most visible at the surface, which is worth bearing in mind for anyone hoping to read the landscape clearly. The monument carries no signage or formal interpretation on the ground, so arriving with Stout's recorded details in hand gives the site considerably more legibility than it would otherwise have.