He was the son, probably the eldest, of King Dumnagual Moilmut and would have succeeded him in the early 6th century. Bran the Old, apparently lived to a great age but, despite this, nothing is known for certain about him. It is quite probable, however, that some elements of his life-story have entered the tale of Belinus & Brennius as told by Geoffrey of Monmouth. This story is an amalgam of various anecdotes concerning several men of similar names, mistakenly or otherwise, merged into a single character. Though transported back to a mythical period around 450 bc, Brennius is clearly named as the son of Dumnagual Moilmut and King of Northumberland (otherwise described as the land north of the Humber to Caithness). The two brothers, central to the story, can, however, also clearly be identified as the gods, Beli & Bran, who appear elsewhere in Celtic myth. The fighting brothers is a popular theme throughout World mythology, but it may be that the details have been influenced by events from the life of the historical Bran Hen. The story goes something like this:
Upon the death of their father, the brothers Belinus and Brennius inherited the Kingdom of Britain. There was some disagreement about the division, but eventually Brennius took the lands north of the Humber and Belinus the rest. While the former was abroad marrying the daughter of the King of Norway, Belinus invaded his kingdom. Brennius tried to return but, through a number of misadventures, lost both his army and his wife. He eventually ended up in Burgundy where he married the monarch's daughter and inherited the kingdom. With a vast army, he invaded Britain but, through the intercession of his mother, was reconciled with his brother.
The later episode involving the conquest of Rome is based on the historical Gaulish leader Brennus. Chretien de Troyes places Bran Hen in his correct chronological context when he refers to Kings Belin & Brien in his Arthurian romance, Erec & Enid. he was succeeded by either his brother, Cuncar, or his nephew, Morcant Bulc.