Cían's father had been killed by Brian Boru at the Battle of Belach Lechta.
He was a member of Uí Echach Muman or Eóganacht Raithlind,[citation needed] and an "ancestor of the family of O'Mahony".
[1] Cían became a close ally of Brian,[2] and reputedly married his daughter Sadb, according to later traditions and the antiquarian John O'Mahony.
[3][4] Several sources suggest that Cian lived at Enniskean in County Cork,[5] and that the village derives its name from him.
[3][4] He had a son, Mathghamhain mac Cian who died at the Battle of Clontarf in 1014.