References in early Irish literature identify Crúachu ("the mounded place") as an assemblage of burial mounds at which an oenach (assembly) was held, and as a dwelling place of the earth goddess Medb and her husband Ailill, both of them rich in cattle. Oweynagat nearby was the gateway to the otherworld in the early tale Echtra Nerai. The site of Carnfree (Carn Fraich) was the inauguration place of the O'Connor kings of Connacht, their ancestors and forerunners, from time immemorial until the fourteenth century. Both these sites are foci around which hundreds of monuments, constructed over at least four millennia, are concentrated.
Fifty mounds I certify,
Are at Oenach na Cruachna,
There are under each mound of them
Fifty truly fine warlike men.
(from a poem in Leabhar na Uidhre [Book of the Dun Cow], quoted with translation in Petrie 1845, p. 104)
The monuments of the Cruachain/Carnfree complex stand on two great ridges 400 to 500 feet above sea level, running northeast to southwest on the ancient plain of Magh n-Aí west of the Shannon in County Roscommon. Dominating the northwest end is the cemetery around Rathcroghan (Ráth Cruachan), a natural mound over 6 meters in height, scarped and built into a roughly circular form 100 meters across, with a circular bank and ditch on top which identify the mound as a ring-barrow. Around it is a ditched enclosure 370 meters in diameter. Within the cemetery complex are Stone Age megalithic tombs, two Early Bronze Age tumuli, and several late prehistoric ring-barrows, with a few tiny round cairns and standing stones. Prominent features of the complex are three parallel avenues running straight along the axis of the ridge, two of them overlain by ring-barrows. Four named wells in the area demonstrate the importance of drinking water for cattle in this prime grassland. On the more southerly ridge 6 kilometers to the southeast stands Carnfree, a burial cairn of the early Bronze Age; it lies close to a classic "bowl barrow" of the same period. A later cemetery of ring-barrows and three standing stones surround these focal tombs.
These twin cemeteries encircle spaces that reputedly were in early historic times an oenach site and an inauguration site. The sacred character of the cemeteries probably led to the creation of a zone of exclusion around each of them, outside of which several ringforts were built between 500 and 1000 c.e. In that era many tales about these sites were written down. The great spring well of Ogulla between Rathcroghan and Carnfree, the most powerful in the area, is pointed out as the meeting place of Saint Patrick and the daughters of King Laoghaire of Tara at the dawn of Christianity in the second half of the fifth century.
Herity, Michael. "A Survey of the Royal Site of Cruachain in Connacht, I." Journal of the Royal Society of Antiquaries of Ireland 113 (1983): 121–142.
Petrie, G. Trans. Royal Irish Acad. xx (1845): 104.