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In Irish mythology, Danu ([ˈdanu]; modern Irish Dana [ˈd̪ˠanˠə]) is the mother goddess of the Tuatha Dé Danann (Old Irish: "The peoples of the goddess Danu"). Though primarily seen as an ancestral figure, some Victorian sources also associate her with the land.
The theonym is of Proto-Indo-European age, and seems to have denoted a water goddess in origin. A goddess Dānu is attested in the Rigveda, and also the river names Danube (Latin: Danuvius), Dniestr, Dniepr and Don derive from the name.
The Rigvedic Danu was the mother of a race of Asuras called the Danavas. A shortened form of the name appears to have been Dā. The Greek goddess Demeter (Da-mater), is also associated with water several times.  Julius Pokorny reconstructs the name from the PIE root da:-: "flow, river", da:-nu: "any moving liquid, drops", da: navo "people living by the river, Skyth. nomadic people (in Rigveda water-demons), fem.Da:nu primordial goddess , in Greek Danaoi (Danaans, Greek tribe, Egypt. Danuna).  
The genitive form of Old Irish Danu is Danann, and the dative Danainn. Irish Danu is not identical with Vedic Dānu but rather descends from a Proto-Celtic *Danona, which may contain the suffix -on- also found in other theonyms such as Matrona, Maqonos/Maponos and Catona.
Danu was considered as the mythic mother goddess of the Tuatha Dé Danann, the Celtic tribes that first invaded Ireland. The Celts, also on the continent, had several goddesses, also of war. "Apart from these goddesses of war, there were other Amazonian figures who led armies into battle. Often they were also endowed with legendary sexual prowess..." "The Celts included the cult of the mother goddess in their rites, as archeological evidence testifies. Indeed, the Tuatha Dé were the descendants of the goddess Danu, and in some local instances, the ruler of the otherworld was a goddess, rather than a god,just as some folktales represented the otherworld as 'the Land of Women'. Danu may be connected with Brigit, daughter of Kildare and of learning, culture and skills, as both of them have been described as daughters of the Dagda at one point and also have been described as mothers of Brian, Iuchar, and Iucharba. Brigit was known as Brigantia in northern England is hypothesized to have survived as St Bride in Christianity.