Seán lived in the parish of Creagh, Ballinasloe, and was described by historian Rev Jerome Fahey as:an exceedingly influential petty-chief, holding a position astride the main highway into western Connacht.
was the wife of Domhnall Ó Madadhan, lord of Síol Anmchadha, the territory to the south of Clonmacnowen.
As a result of the strategic importance of his lands, and their extensiveness, he was sought out by the Castle Administration in Dublin, who hoped to make him abandon the Gaelic Brehon laws and become a loyal subject of the crown.
In an effort to secure his lands from other Ó Cellaighs, and to obtain the protection of the crown, Seán made a submission of Surrender and regrant in 1578, acknowledging the rule of Elizabeth I.
This grant to exonerate the grantee from any composition made with the lord deputy for the support of the Soldiers in the province – 26 July (1578)Many of these lands later became the town of Ballinasloe and its environs; its castle was built and occupied by Edward Brabazon, 1st Baron Ardee.