The name belongs to a widespread Irish tradition of attaching the story of the fugitive lovers Diarmuid and Gráinne to megalithic tombs across the country, as if the monuments needed a human story to explain their presence in the landscape. The tomb itself is considerably older than any such legend, and considerably more architecturally deliberate.

This is a wedge tomb, a type of megalithic monument built during the late Neolithic and early Bronze Age, generally between roughly 2500 and 2000 BC. The defining feature of the form is structural: the cairn, the mound of stones covering the burial chamber, is broader and taller at the entrance end and narrows progressively towards the rear, giving the whole thing its wedge profile when seen from above or from the side. At Carrowcrom the cairn is roughly U-shaped in plan, about six metres long and up to six and a half metres wide at its western end, where it reaches a maximum height of one and a half metres. The entrance, set into the centre of the flat south-western façade and flanked by two carefully matched jamb stones, leads into a gallery just under three and a half metres long. Four large upright stones, or orthostats, line each side of this inner chamber, and two massive overlapping capstones form the roof overhead. An outer wall of upright stones was also constructed around the monument, though most of it is now buried within the cairn; a few of those outer stones can still be seen protruding at the rear. The tomb is a National Monument in state ownership, catalogued as no. 293.

The site sits on level pasture ground, but the surrounding terrain is undulating, and fifty metres to the east the land drops sharply into a depression of boggy ground. The Ox Mountains fill the skyline to the east-south-east, and on a clear day Nephin Mountain, the isolated quartzite peak that rises above the Mayo boglands, is visible on the far horizon to the west-south-west. Whatever the original builders understood about the landscape, they chose a position with a long view.