Local man Henry Morris was excavating what appeared to be an ancient mound when he uncovered a stone chamber measuring approximately 10 feet long by 7½ feet wide, built from carefully placed stones. Among his finds was a stone axe, though it seems to have been disturbed from its original position near the top of the mound, making it difficult to determine its exact age or purpose.

The mound itself is a modest but intriguing feature of the local landscape. Circular in shape, it measures 8 metres north to south and 6 metres east to west, rising just over a metre in height. Time hasn't been entirely kind to the structure; its eastern edge was removed at some point to make way for a gateway, and it sits immediately north of a roadside fence. When archaeologists revisited the site in 1994, they found it overgrown and challenging to examine properly.

What makes this site particularly mysterious is the uncertainty surrounding its original purpose. While some stones visible on the mound's surface appear to have been deliberately placed, their age and function remain unclear. Archaeologists have noted that these stones seem to be set within the mound itself rather than on the original ground surface beneath, which suggests they weren't part of a megalithic chamber typical of prehistoric burial sites. This detail, recorded in the Survey of the Megalithic Tombs of Ireland, leaves the true nature of the Mullanacross mound an open question, adding another layer of intrigue to Donegal's rich archaeological landscape.