Rathcroghan (Irish: Ráth Cruachan, meaning 'ringfort of Cruachan') is a complex of archaeological sites near Tulsk in County Roscommon, Ireland. It has been identified as the site of Cruachan or Cruachu, the traditional capital of the Connachta, the prehistoric and early medieval rulers of the kingdom of Connacht. Rathcroghan is one of the royal sites of Ireland. The ritual landscape extends over 6 square kilometres (2.3 sq mi) and consists of over 240 archaeological sites, 60 of which are protected national monuments. According to National Geographic, the site may be "Europe's largest unexcavated royal complex".
The sites range in time period of construction from the Neolithic (4000–2500 BC), the Bronze (2500–500 BC) and Iron Ages (500 BC – 400 AD), to the medieval period. These monuments include burial mounds, ringforts and medieval field boundaries, the most prominent sites being the multi-period Rathcroghan Mound, the Oweynagat cave, the Mucklaghs (a set of linear earthworks), and the Carnfree medieval complex.
There are many historic references to Rathcroghan (Ráth Crúachan) recorded in early medieval manuscripts, including the 12th-century Lebor na hUidre. Rathcroghan is recorded as the location of one of the great fairs of Ireland, as well as being one of the island's three great pagan cemeteries. It is also the location for the beginning and end of the Táin Bó Cúailnge and the royal seat of Medb, a mythical queen of Connacht.
The Cave of Cruachan, also called Oweynagat, is said in Irish mythology to be an entrance to the Otherworld, or in the modern era as one of Ireland's "Gates to Hell". The cave has associations with the festival of Samhain and is also described as the dwelling of the Morrígan, a mythical figure in early medieval Irish literature.
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