Wikipedia
Mongfind (or Mongfhionn in modern Irish)—meaning "fair hair" or "white hair"—was the wife, of apparent Munster origins, of the legendary Irish High King Eochaid Mugmedón and mother of his eldest three sons, Brión, Ailill and Fiachrae, ancestors of the historical Connachta, through whom she is an ancestor of many Irish and European nobility today. She was the sister of Crimthann mac Fidaig, King of Munster and the next High King of Ireland, whom she is said to have killed with poison in the attempt to have the kingdom for her sons. She drank the same to convince him, and died soon after at Samhain, becoming a goddess of sorcery.
She was the first wife of Eochaid; he took a second wife, Cairenn, who gave birth to Niall of the Nine Hostages. Several stories depict Mongfind as an adversary of her stepson.
According to Cormac's Glossary, she was a goddess the pagan Irish worshipped on Samhain. This was also called the Féile Moingfhinne i.e. "Festival of Mongfind". Later legend, as documented in Patrick Weston Joyce's Social History of Ancient Ireland, makes her a bean sidhe. A prominent hill called Cnoc Samhna "Hill of Samhain" also known as Ard na Ríoghraidhe "Height of the Kingfolk" south of Bruree, County Limerick is associated with a tale connected to Mongfind. "Anocht Oíche Shamhna Moingfhinne banda" is children's rhyme from County Waterford which translates as "Tonight is the eve of Samhain of Mongfhionn the goddess". Variant spellings of her name include Mongfinn, Mongfionn, Mongfhind, Mongfhinn, Mongfhionn, Mongfinne, Mongfhinne, Mong Find, Mong Finn, Mong Fionn, Mong Fhionn, Mungfionn, Mung Finn, Mung Fionn.
Mongfind and her brother, children of Fidach and grandchildren of Dáire Cerbba in most sources, are sometimes said to belong to an early or peripheral branch of the Eóganachta. However, this is unlikely, as the evidence suggests that, if historical, they belong to a distinct people associated with other kingdoms, possibly the Dáirine, who may be referred to as their people in an obscure poem in Old Irish by Flann mac Lonáin (d. 896). In the Banshenchus she is called "Mongfind of the Érnai" (Érainn), and given a later son Sidach following the Connachta. Dáire Cerbba is stated in Rawlinson B 502 to have been born in Mag Breg (Brega), Mide, much of which probably remained Érainn territory at the time of his supposed floruit. It is difficult to distinguish the Dáirine from the Érainn in the surviving corpus.