Marked simply as ‘Carn’ on the 1834 and 1848 editions of the OS 6-inch maps, this feature was later identified by researcher Colhoun in 1949 as a multi-chambered cairn, a type of prehistoric burial monument typically consisting of multiple stone chambers covered by a mound of stones. These monuments, dating back thousands of years, were built by Ireland’s Neolithic communities as elaborate tombs for their dead.

Today, however, visitors to the site would find no trace of this ancient structure. Whatever once stood at this location has vanished entirely, leaving behind neither stones nor earthworks to hint at its former presence. The cairn’s disappearance remains something of a mystery; it may have been dismantled for building materials during the 19th or early 20th century, a common fate for many of Ireland’s prehistoric monuments, or it could have been gradually destroyed through agricultural activities as fields were cleared and improved.

The phantom cairn of Knockglass serves as a reminder of how much archaeological heritage has been lost over the centuries. Its inclusion in multiple archaeological surveys and records, from Bonner’s work in 1972 through to the Record of Monuments and Places in 1995, ensures that even though the physical monument has disappeared, its memory persists in Ireland’s archaeological record. The site was formally documented in the Survey of the Megalithic Tombs of Ireland, compiled by Eamon Cody and published by the Stationery Office in 2002, preserving what little information remains about this lost piece of Donegal’s prehistoric landscape.