Located 220 metres west of a cruciform passage tomb known as Site A, this lost cairn was documented by Thomas Fagan between 1845 and 1848 as a ruined structure approximately 23 metres in diameter, enclosed by stone blocks that likely formed a kerb. Within the cairn, Fagan discovered an intriguing array of graves; square, oblong and circular in shape, some covered with stone flags bearing what he described as 'sundry rude devices'. Human bones were reportedly found within these graves, suggesting this was indeed a burial site of some significance.

The monument's proximity to other passage tombs in the Kilmonaster cemetery complex hints that it too may have been a passage tomb, though some archaeologists have proposed it might have been a cemetery mound containing stone cists instead. Unfortunately, the site's removal means we'll never know for certain what secrets it held. The area continues to yield occasional archaeological finds; in 2005, pre-development testing for agricultural buildings just five metres from another passage tomb in the complex uncovered a worked flint in the topsoil, a small reminder of the prehistoric activity that once took place here.

Today, the Kilmonaster passage tomb cemetery remains an important archaeological complex in Donegal, though this particular monument exists only in historical records. The site was officially recorded in the Survey of the Megalithic Tombs of Ireland and given the designation 70:63 in the Record of Monuments and Places, preserving at least the memory of what was once a significant feature of Ireland's prehistoric landscape.