Dolmens were usually covered with earth or smaller stones to form a barrow, though in many cases that covering has weathered away, leaving only the stone "skeleton" of the burial mound intact. There is evidence to suggest that the earliest portal tombs may have had timber prototypes.
They are thought to be related to the Court Cairn, another type of megalithic tomb from the early Neolithic period (c. 4000 BC), and are mostly found in the northern half of the country, although significant clusters can be found in a zone from Co. Dublin to Waterford, and the Clare/Galway region. Perhaps the most famous example is in Poulnabrone, in the Burren in Co. Clare. They are most often found in low lying land, and when excavated are found to contain cremated remains, and burial goods such as pottery and flint tools.
This type of tomb can also be found in other countries on the western Atlantic such as Wales and Cornwall, and goes by a variety of names, such as Cromlech, Anta, Hünengrab, Hunebed, Goindol and Quoit.