He was of the Sil Máeluidir sept of this branch of the Laigin, who were found in the later baronies of Shelmalier on the lower reaches of the Slaney River in southern modern County Wexford.
The date of Bran's accession is unknown but the Book of Leinster king list gives him a reign of three years which would correspond to 709–712.
His last paternal ancestor to hold the kingship was Éogan Cáech mac Nath Í who would have lived in the early 6th century of whom Bran was a 5th generation descendant.
The last member of this sept to hold the throne was Forannán mac Máel Udir whom the king lists would place in the late 6th century.
One of Bran's sons did survive, Cennselach mac Brain (died 770) who later became king of the Uí Cheinnselaig.