Cían is identified as the king of Magh Senchineol, home to the Senchineoil people, who were said to be of Fir Bolg origin, a population group from Irish mythology.
Mac Fhirbhisigh states that Aonghus led his people, the Tuath mhac nUmhoir, to the coast of Galway Bay and the Aran Islands, after being driven out by warfare with "Clann Chuian and the kindred of the Gaoidhil (Gaels)."
According to Leabhar Ua Maine: These fine hosts suddenly and heroically proceeded in well arranged battalions, with their flocks and herds, from Clochar Mac Daimhin to Druim Clasach, which is called Tir-Many, situated between Loch Ri and the river Suca.
Mac Fhirbhisigh quotes a quatrain which describes the army: Aoinfhear as gach lios amach/as eadh do thigeadh le Cian/a Maigh Sencheneiol, ni breug/dech cceud are fhichit ceud sgiath.
Grellan, from the door of his church, perceived these arms, and these great hosts, he raised his two hands to God, being apprehensive that his guarantee would be violated, and he obtained his request from God, for the great plain was softened and made a quagmire under the feet of Cían and his people, so that they were swallowed into the earth; and the place received the name of Magh Liach, i.e. the plain of sorrow, from the sorrow of the heroes, who were thus cut off by the holy cleric.
Mac Fhirbhisigh wrote of him "Although this Cían was notable at that time, there is scarcely any little thing known of the flower of his kindred because of the intermingling of the family of the Collas with it until now.