Domnall (d. 1105), abbot of Armagh, was a son of Amalgaid and a member of the ecclesiastical lineage of Clann Sínaig (descended from the Uí Echdach lineage of Airgialla), which virtually monopolised the headship of Armagh in the eleventh and twelfth centuries, its representatives frequently being styled airchinnech (lay–abbot). Although his father Amalgaid (qv) had died as airchinnech of Armagh, Domnall's immediate predecessor in that office was his brother Máel-Ísu (d. 1091).
Domnall headed the Armagh community for fourteen years, as attested by the list of coarbs (successors) of St Patrick (qv). During that time, he intervened succesfully on no less than six occasions to prevent battle between Domnall Ua Lochlainn (qv), king of the North, and the ambitious Munster overking Muirchertach Ua Briain (qv), claimant to the high-kingship of Ireland. In 1103 Muirchertach spent a week camped at Armagh and offered eight ounces of gold and eight score cattle in an effort to secure the support of Abbot Domnall, but the latter was unwilling to endorse the claims of the Munster king. Domnall, styled heres Patricii (heir of St Patrick), died at Armagh in 1105 at an advanced age. He was succeeded by his distinguished great-nephew Cellach (qv) son of Áed, who was to make a major contribution to church reform.