Question 5 of 30 Days of Deity Devotion
Fionn is the son of the warrior Cumhall, who is said to have died before his birth. His mother is Muirenn Munchaem”of the white neck” the daughter of the druid Tadgh mac Nuadat, who was kidnapped and impregnated by Cumhall. He may have been killed in vengeance by Tadgh.
Tadgh is the son of Nuadu Airgetlamh (of the Silver Arm) a king of the Tuatha De Dannan, or he may himself be a form of Nuadu.
Muirenn could not raise Fionn, so he was nursed by her sister, the ban-draoi Bodhmall. His foster-father was Fiacclach mac Conchinn, his son Moling Luath is his foster brother. Two bio-brothers are named as Fithel and Feinnidh, his aunt or sister is Uirne, wife of Illann and mother of Bran and Sceolang.
While the earlier hero Cuchulainn for the most part sticks to Emer, Fionn has many lovers and wives. The one that stands out the most is Sadb, who is turned into a doe and gives birth to his son, Oisin as a fawn. The other important wife would be Ailbe, daughter of his patron, Cormac mac Airt. Other wives: Berrach (called his third wife) Cruithne, Daolach, Maigneis, Smirgat, Taise and Teite. Aine (seems to be different than the love goddess of Cnoc Aine) would sleep with no other man, and in one version he fathers two sons with her, in another he rejects her.
Sons: In addition to Oisin (who marries Niamh and has Oscar) there is Cairelle (killed by Goll), Daire, Faelan mac Finn, Fergus Finbel, Fiachra and Fiachna.
Daughters: Ai Arduallach the arrogant, Cainche, mother of Goll’s children, Lugach, foster-daughter: Bebinn.
Reference: Oxford Dictionary of Celtic Mythology by James MacKillop