Cruachan (Cruachu, Cruachain, Ráth Cruachain) is the ancient capital of the kingdom of Connacht, and the seat of Medb and her husband Ailill of Connacht in the Ulster Cycle of Irish mythology. It is the setting for the opening section of the Táin Bó Cúailnge. Its site is now known as Rathcroghan, a low, flat-topped mound surrounded by a complex of archeological sites near Tulsk in County Roscommon. A standing stone there is said to mark the grave of Dathí, one of the last pagan High King of Ireland.
Other sites in the Rathcroghan complex include Rath na Darbh, a large circular enclosure, traditionally supposed to be where the bulls Donn Cuailnge and Finnbhennach fought at the end of the Táin Bó Cúailgne. Reilig na Rí is the name given to a low circular ditch whose inner area many believe is divided to resemble an ancient map of Ireland. Near this monument are the Mucklaghs, two long linear enclosures with banks up to 6m high, similar to the Banqueting Hall at Tara. Traditionally believed to be dug out of the ground by a magical boar, there is a view that these structures may have been ceremonial buildings or to store and protect animals. On the other side of the N5 roadway lies Rathmore, probably the finest looking example of a Celtic dwelling on the whole site. Nearby this lie the Rathscreig and Flanagans Ring barrows that cut on to an earlier man made roadway known as the Ancient Avenue. Perhaps the most famous feature in the area aside from Rathcroghan mound is Owenagcat, a cave which can be entered by going through an old souterrain. It is a narrow cave, and many mythical creatures are said to have emerged from here along with the war goddess Morrígan.
Cruachain seems to have heavy associations with the feast of Samhain, as it was during this time that the Irish believed that the prehistoric graves from before their time opened and their gods and spirits, who dwelt inside, walked the earth. The emerging of creatures from Owenagcat would be part of this belief. A legend based on this is The Adventures of Nera, in which the warrior of the title is challenged to tie a twig around the ankle of a condemned man on Samhain night. After agreeing to get some water for the condemned man he discovers strange houses and when he finally gets him some water at the third house. He returns him to captivity only to witness Rathcroghan's royal buildings being destroyed by the spirits. He follows the fairy host to the sidhe where he meets a woman who tells him that what he saw was a vision of what will happen a year from now unless his mortal comrades are warned. He leaves the Sídh and informs Ailill of his vision who then has the Sidhe destroyed. It is unclear whether what is referred to as the Sídh is Owenagcat or the mound of Rathcroghan itself.
Only geophysical surveying has really been done in the area, primarily on the mound. This reveals signs of enclosure rings on its summit, along with evidence of a trench, 340 m in diameter, which had the mound as its centre.