The discovery at Lisnamulligan prompted an immediate archaeological investigation by O'Connor from the National Museum of Ireland, revealing a carefully constructed Stone Age tomb that had been undisturbed for thousands of years.

The cist itself is a compact, slightly wedge-shaped chamber measuring just 0.84 metres from east-northeast to west-southwest and 0.38 metres from north-northwest to south-southeast, with a depth of 0.4 metres. Its construction shows remarkable craftsmanship; six stone slabs were set on edge to form the walls, with one slab on each long side and two stacked slabs on each of the shorter ends. The southern side slab appears to have extended beyond the western wall, suggesting deliberate architectural planning. A single stone slab formed the floor, whilst the massive capstone, measuring 1.32 metres by 0.78 metres and 0.27 metres thick, had been displaced from its original position when found.

Although the local Gardaí had unfortunately removed some earth from the cist's interior before archaeologists arrived, the excavation still yielded human bones from an ancient inhumation burial, though no grave goods or artefacts were recovered. The plot thickened when a second cist was discovered approximately three metres to the southwest, hinting that this quiet Donegal hillside may have served as a burial ground for an ancient community whose other traces have long since vanished from the landscape.