Flann Sinna (‘of the Shannon’) (d. 916), son of Máel-Sechnaill and king of Tara, was a member of the Clann Cholmáin dynasty of the Southern Uí Néill. His father Máel-Sechnaill (qv) had died as king of Tara in 862; his mother was Lann daughter of Dúngal of the Dál mBirn dynasty of Osraige. He apparently had siblings, but details are lacking. His wives included Gormlaith, daughter of the Síl nÁedo Sláine ruler Flann son of Conaing, and Máel-Muire, daughter of Cináed (qv) son of Ailpín, king of Dál Riata and king of the Picts. The former was the mother of his son Donnchad Donn (qv) and of his daughter Gormlaith (qv), who married Niall Glúndub (qv) of Cenél nÉogain. The latter was the mother of his daughter Lígach (d. 923), who in turn married Máel-mithig, the Síl nÁedo Sláine king of Brega, and bore him a son, Congalach Cnogba (qv). Flann had six other sons whose mothers are not recorded: Máel-ruanaid (slain 901), Óengus (d. 915), Conchobar (slain 919), Domnall (slain 921), Cerball, and Áed.
Flann did not succeed his father directly as Clann Cholmáin king of Mide. Some confusion prevailed within the dynasty in the mid 860s, and the king-lists record one Lorcán son of Cathal and Donnchad son of Eochucán; Flann slew the latter, who seems to have been a second-cousin, and made good his own claim to the kingship. He needed to secure, and later maintain, relationships with his Síl nÁedo Sláine rivals to the east – his marriage to Gormlaith should perhaps be viewed in this context. The death of the king of Tara, Áed Findliath (qv), in 879 enabled Flann to claim the paramount kingship of the Uí Néill. He celebrated the Óenach Tailten (fair of Tailtiu; Teltown, Co. Meath), and then spent the next fifteen years or so consolidating his position. He apparently had little compunction about joining with the Scandinavians of Dublin to move against the Northern Uí Néill and attack their principal church of Armagh, although in 888 he suffered a setback when his erstwhile Norse allies turned against him.
From Flann's perspective, the death in 888 of the powerful Osraige king Cerball (qv) son of Dúngal, who had overshadowed Leinster and Dublin, reopened the prospect of asserting Uí Néill influence in the south of the country. He reacted decisively when Cerball's son Diarmait emerged in 894 and dared to celebrate the Óenach Carmain (Fair of Carman, east of Kilcullen, Co. Kildare), a prerogative of the king of Leinster. Forming an alliance with the Uí Fháeláin claimant to overkingship of Leinster, Cerball (qv) son of Muirecán, Flann defeated the troublesome king of Osraige.
In the years that followed, Flann continued to develop the relationships that his predecessors had initiated with the ecclesiastical centres of Clonard and Clonmacnoise. On occasion, he was able to turn such relationships to his political advantage. When the Connachta, under their king Cathal (qv) son of Conchobar, raided the Shannon church-site of Inis Aingin in 900, Flann apparently had the support of the Clonmacnoise community in securing a rígdál (royal meeting) with his opponent. As a result, Flann secured the allegiance of the dynasty of Uí Briúin Aí. There was opposition from within Meath itself, however, his son Máel-ruanaid being slain by the Luigne in 901. Similarly, there was a threat from the Connacht–Ulster marchland dynasty of Uí Briúin Bréifne; later (910) he was obliged to repel an invading force from this quarter at Druim Criaich (Drumcree, Co. Westmeath).
Meanwhile, Munster had proved increasingly hostile to Flann's policies – especially with the emergence of the ambitious Cormac (qv) son of Cuilennán. Punitive expeditions by Flann in 905 and again the following year, in which he took hostages, failed to curb the growing power of Cormac. In fact the latter managed to defeat the Uí Néill at Mag Léna (907). Flann's reply was to resurrect his alliance with the Leinster king Cerball son of Muirecán: in 908 the two dealt a fatal blow to Cormac at the battle of Belach Mugna (barony of Idrone, Co. Carlow), where many of the nobility of Munster fell. Now at the height of his power, Flann reinstated his one-time opponent Diarmait son of Cerball as king of Osraige in the place of his dead brother Cellach who had supported Cormac in the battle.
It may have been at this stage, out of gratitude for his great victory perhaps, that Flann commenced to generously endow Clonmacnoise. The appointment of Colmán (d. 926) son of Ailill of the Conaille, abbot of Clonard, to head the community of Ciarán (qv) in duality from 904 may have originated with Flann. In 909 the king and Abbot Colmán collaborated on the building of a ‘great stone church’ at Clonmacnoise. The elaborate Cross of the Scriptures has a partially illegible inscription which seems to request prayers for Colmán and Flann and to suggest that the latter was responsible for the erection of the cross. Flann presumably understood that his role as church benefactor would legitimise his dynasty's influence over ecclesiastical government. After his death, two clerics of the Uí Maic Uais of Meath held abbatial office at Clonmacnoise; one was in fact a transfer from Clonard. One of his grandsons, Donnchad (d. 923), was vice-abbot at the latter foundation.
Flann's later years were not free from dynastic strife; in 904 he was forced to suppress a revolt by his son Donnchad Donn, while the latter and his brother Conchobar both rebelled in 915. The ageing Flann only survived this challenge with military support from his son-in-law, Niall Glúndub, who had hitherto defied his authority. After Flann's death, which took place the following year on 25 May (at 7 a.m., the annalist notes), tensions between his sons soon emerged. Conchobar immediately succeeded his father, but was slain at the battle of Dublin (919); his successor, his brother Domnall, was soon slain by Donnchad Donn, who then reigned for twenty-five years.