What was the name of Fionn mac Cumhaill's father?

The name of Fionn mac Cumhaill’s father was Cumhall, also called Cumhall mac Trénmhoir.

Cumhall is a leader of the fianna in the Fenian Cycle of Irish mythology. He served the High King Conn Cétchathach, or Conn of the Hundred Battles, and was associated with Clan Bascna, the group to which Fionn belonged. The name “mac Cumhaill” means “son of Cumhall.”

According to the story known as Fotha Catha Chnucha, Cumhall sought to marry Muirne, daughter of the druid Tadg mac Nuadat. Tadg rejected the match, and Cumhall carried Muirne away. Their conflict eventually led to the Battle of Cnucha, where Cumhall was killed by Goll mac Morna, who then took leadership of the fianna.

Muirne was already pregnant, and she later gave birth to the child who was initially named Demne or Deimne. That boy became Fionn, the legendary leader of the Fianna. Goll is therefore a rival figure, while Conan and Oisín are associated with Fionn’s later adventures.