She is said to have been the wife, of apparent Munster origins, of the legendary High King Eochaid Mugmedón and mother of his eldest three sons, Brión, Ailill and Fiachrae, ancestors of the historical Connachta.
"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".
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.
[7] Dáire Cerbba is stated in Rawlinson B 502 to have been born in Mag Breg (Brega), Mide,[8] much of which probably remained Érainn territory at the time of his supposed floruit.
[9][10] In this she is associated with sites belonging to her relatives the Uí Fidgenti,[10] although they are not specifically mentioned, and neither is Crimthand Mór, her brother in the other tales.
She is placed in the alternate version encamped at Cnoc Samhna (Knocksouna) i.e. Ard na Ríoghraidhe,[10] which may have been the inauguration site of the Uí Fidgenti.
Later tradition finds them in alliance with Mongfind's descendants the Uí Fiachrach Aidhne, especially in the time of Guaire Aidne mac Colmáin, king of Connacht.
In one and perhaps the oldest version of Mongfind's pedigree the Uí Liatháin are close kin, who are broadly associated with Crimthann mac Fidaig in other sources.
Theirs is said in several early tales to have been a sister kingdom to the Uí Fidgenti mentioned above, which the genealogies confirm, but modern scholarship dismisses the two as 8th century add-ons to the Eóganachta pedigree,[13] which may have implications for the ancestry of Mongfind and Crimthann.