Flann Sinna [Flann mac Máele Sechnaill] (847/8–916), high-king of Ireland, was the son of , high-king of Ireland, and of Fland (d. 890), daughter of Dúngal mac Fergaile, king of Osraige. A member of the Clann Cholmáin, the main branch of the southern Uí Néill dynasty based at the crannog of Cró-inis in the south-west corner of Lough Ennell in Westmeath, he became king of Mide in 877 and succeeded to the high-kingship of Ireland in 879 on the death of the northern high-king, Áed Findliath.

Flann had seven sons and three daughters from three marriages. He was married to Máel Muire (daughter of Cináed mac Ailpín, king of Scots [see ]), who died in 913; she was the mother of Domnall who was killed by his brother Donnchad in 921, and of Lígach (wife of Máel Mithig mac Flannacáin, king of Brega—a branch of the dynasty at Knowth, east Meath), who died in 923 and was buried at Clonmacnoise. With Gormlaith, daughter of Flann mac Conaing, king of all Brega, he had , who died in 944 as high-king, , and possibly Conchobor, who died in the battle of Cell Mo Shámóc (Islandbridge, Dublin) on 15 September 919. Conchobor and Donnchad rebelled against their father from a Brega base in 915. Flann Sinna's marriage to Eithne (daughter of Áed Findliath), who died on 11 November 917, produced Máel Ruanaid who was burned to death in a house by the Luigni of Meath in 901. His other children, whose mothers are unknown, were three sons: Óengus, who died on 7 February 915 from a wound sustained in an encounter with the northern Uí Néill on 10 December 914; Áed, blinded by his brother Donnchad in 919; and Cerball (date of death unknown); and also a daughter, Muirgel, who died at Clonmacnoise in old age in 928. She had conspired in the death of a son of the viking king Auisle in 883. Her accomplice was Otir, son of Iergne (Járnkné), a member of the Find-Gennti or Norwegians. This may suggest that she was married to a viking leader at Dublin.

Flann is first noted in 877, killing the king of Mide, Donnchad, by deceit. His ruthlessness may be further seen with the burning in 891 of the king of Uí Failgi in Clonfad, a few miles south of Cró-inis, while that king was returning from a conference with Flann; and in his instigation of a killing at Trevet in Meath by his ally Máel Mithig and his son Óengus in 903. In 880 Flann took the hostages of Leinster and then plundered Munster. In 882 he campaigned in the north and in Munster. Despite a defeat by the Dublin Norse in 888, campaigns against him by the northern Uí Néill in 889 and in December 914, and opposition from his sons Máel Ruanaid in 898, Donnchad in 904, and Donnchad and Conchobor in 915, Flann maintained his position as high-king. In 897 he turned his attention to Connacht, gaining the submission of its king in 900. He kept south Brega in check through an alliance with the branch of the dynasty based at Knowth. Munster, however, was a constant source of opposition. Despite a victory for the men of Munster in 907 they were heavily defeated and their king, Cormac mac Cuilennáin, was killed at the battle of Belach Mugna (Ballymoon, just east of Muine Beag in modern co. Carlow) on 16 August 908. This confirmed Flann as king of Ireland. He was responsible for the erection of the Cross of the Scriptures (the inscription on the west face of which describes him as rí hÉrenn, ‘king of Ireland’) and the damliag, the ‘cathedral’, in 909 at Clonmacnoise—his likely burial-place. Flann Sinna died, aged sixty-eight, on Saturday 25 May 916 at ‘Cenn Eich’ (location uncertain but probably in Westmeath, near the Shannon) on Clonmacnoise property.

Charles Doherty