The cairn, a roughly circular mound of granite stones fifteen metres across and still standing somewhere between one and one and a half metres high, predates its modern neighbour by several millennia. A cairn of this type is essentially a burial or ceremonial mound constructed from loose stones rather than earthen material, and this one was built from the same granite that makes up the mountain itself, each stone running to roughly forty by twenty centimetres.

The two structures tell a quiet story about how summits accumulate meaning over time. People were marking this peak long before anyone needed to triangulate the landscape of County Wexford. The trigonometric station, erected after 1945, disturbed the cairn in the process of asserting a more recent kind of significance on the same spot. The Blackstairs Mountains run on a northeast to southwest axis along the Wexford and Carlow border, and Mount Leinster is their highest expression, making it an obvious candidate both for prehistoric monument builders and for twentieth-century surveyors seeking clean sightlines across the country.