Cumhall (earlier Cumall, pronounced roughly "Coo-al" or "Cool") son of Trénmór ("strong-great") is a figure in the Fenian Cycle of Irish mythology, a leader of the fianna and the father of Fionn mac Cumhaill.
Cumhall was a suitor for the hand of Muirne, daughter of the druid Tadg mac Nuadat, future mother of Fionn mac Cumhaill, but Tadg refused him, so he and Muirne eloped. Tadg appealed to the High King, Conn of the Hundred Battles, who made war against Cumhall. Cumhall was killed in the Battle of Cnucha, as recounted in the Middle Irish tale Cath Cnucha, by Goll mac Morna, who took over leadership of the fian, but Muirne was already pregnant with his son, Fionn. Cumhall is reputed to be buried within the grounds of Castleknock College, (an all-boys secondary school in Dublin 15), supposedly under a hill upon which an old water tower now stands.
Attempts to connect Cumhall with Camulus, a Celtic god of war known from Roman commentary, are now largely rejected. Old Irish cumal means "female slave", and it is possible that a noble father was invented for Fionn to obscure an ignoble origin. Alternatively, some early texts call the hero "Fionn mac Umaill", which may indicate his father's name was originally Umall.
Cumhall had a brother, Crimmal, who was an ally of Fionn.