It was recorded, measured, and noted in official correspondence over ninety years ago, and then, in a sense, it disappeared. Not demolished, not excavated, not explained; simply unlocated, sitting somewhere in the landscape with a diameter of roughly six metres and a height of around two metres, waiting for someone to look in the right place.

What we know comes from a single piece of correspondence, dated the 17th of July 1930, addressed to Dr Adolf Mahr at the National Museum of Ireland and preserved in the Topographical files held there. Mahr, who served as Director of the NMI during the 1930s, was a central figure in Irish archaeological administration of the period, and the Topographical files represent a long-running effort to document monuments across the country, often through letters from local observers, clergymen, or interested landowners. The note describes the mound in plain terms: a circular earthwork, about six metres across and two metres high. Mounds of this general type, low and rounded, could represent any number of things, from a prehistoric burial cairn to a later medieval feature, and without excavation or a firm location it is impossible to say which this might be. The record was compiled as part of the national Sites and Monuments inventory by Geraldine Stout, and uploaded in August 2011, but the entry carries the frank admission that the exact location of the monument is unknown.

Castleknock today is a well-developed suburban area, and any surviving earthwork in the vicinity would likely sit on private land or within one of the older estates on the edge of the Phoenix Park. There is no publicly accessible site to visit, no marker, and no agreed coordinates. The mound, if it survives at all, is the kind of feature that could easily be mistaken for a natural rise in a garden or a field. Anyone with a particular interest in the area might consult the NMI Topographical files directly, or check the National Monuments Service database, where the record sits with its uncertainty intact. It is a reminder that the archaeological map of Ireland is far from complete, and that even a feature within a few kilometres of the capital can remain genuinely, stubbornly unresolved.