The Fenian Cycle collection of prose tells us that Cumhall (pronounced “Cool” / “Coo-al” and also recorded as Cumall), son of Trénmór/Tréanmór was the chieftan of Clann Baiscne of Leinster and also was leader of the fianna, an army who served the High King of Ireland, Cormac mac Airt. His story is interwoven with the legend of Finn McCool, bonded by blood. Cumhal was a great warrior chieftan known for great judgement and perception, and held the treasures of the Fianna, including the magical artefact the stork-skin Oxter bag. It was said that whenever Cumhaill needed something, he simply had to reach into the bag and there it was.
The Finna were a roaming band of warriors, living in the forest during the summer and houses in the winter. They were made up of clans across Ireland, the two main ones being Clan Baiscne led by Cumhail, and Clan Morna led by Goll Mac Morna, the most powerful warrior in the Fianna who resented and challenged Cumhal’s position as leader.
One winter tensions were high, and Cumhal and clan Baiscne stayed with the Druid Tadg mac Nuadat, with Clan Morna staying with the King Conn Cead Cathach. The druid had a beautiful daughter called Murine, who her father spurned every suiter after a prophecy foretold her marriage would cost him his home of the hill of Almu. Over the course of the winter she captured the fearless warriors heart, and he asked for her hand in marriage.
The Druid refused, so driven by love Cumhail kidnapped her and they eloped together. Outraged at this action, Tadg went to King Conn of the Hundred battles, with Clan Morna more than happy to chase them down and fought the Battle of Cnucha (Castleknock) as told in the Irish tale Cath Cnucha.
During the battle, Goll Mac Morna killed Cumhaill to take leadership of the Fianna and the treasures of the Fianba. Tadg mac Nuadat was reunited with his daughter Muraine, and was outraged and shamed to discover she was pregnant. Conn stopped him burning her at the stake and sent her to live with Cumhal’s sister, the druidess Bodhmall who raised her son Deimne, later known as Fionn mac Cumhaill or the anglicised version of Finn McCool. His son would later win back leadership of the Fianna for Clan Baiscne, by slaying Allien at the Hill or Tara during Samhain.
Early texts call the hero “Fionn mac Umaill”, which would suggest his father’s name was originally Umall, but there is scarce mention of this name. Also Cumhall is said to have had a brother, Crimmal, who was an ally of Fionn but I can find little mention of him to validate this either.
The story focuses on the battle between two rival clans of the Fianna, ending in the death of Cumhall of Clan Baiscne (father of Fionn mac Cumhaill) at the hands of Goll mac Morna of Clan Morna.