'Flann of the Shannon'; Irish: Flann na Sionainne), was the son of Máel Sechnaill mac Máele Ruanaid of Clann Cholmáin, the leading branch of the Southern Uí Néill.

Flann's reign followed the usual pattern of Irish High Kings, beginning by levying hostages and tribute from Leinster and then to wars with Munster, Ulster, and Connacht.

However, rather than the military and diplomatic successes of his reign, it is his propaganda statements, in the form of monumental high crosses naming him and his father as kings of Ireland, that are exceptional.

Flann may have had the intention of abandoning the traditional succession to the kingship of Tara, whereby the northern and southern branches of the Uí Néill held the kingship alternately, but such plans were thwarted when his favoured son Óengus was killed by his son-in-law and eventual successor Niall Glúndub, son of Áed Findliath, on 7 February 915.

In the following twenty years, raids by Vikings—called "Foreigners" or "Gentiles" in Irish sources—were small in scale, infrequent and largely limited to the coasts.

[6] The making of a kingship of Ireland, as kings from Flann to Brian Bóruma, Muircheartach Ua Briain and Tairrdelbach mac Ruaidri Ua Conchobair (Turlough O'Connor) exercised, may owe as much to the threat raised by Feidlimid mac Crimthainn, of the Eóganachta of Cashel (Eóganachta Chaisil), King of Munster, as to the Viking raids on Ireland.

Drawing on the support of the clergy of Cashel as well as his own military might, Feidlimid is said by Munster sources to have made himself King of Tara.

Although he was defeated in 841 in battle with Niall Caille of the Cenél nEógain, the High King according to some, Feidlimid's achievements were exceptional.

[9] On Niall Caille's death in 846, the kingship of Tara passed to Flann Sinna's father Máel Sechnaill.

Feidlimid died in the following year, and Máel Sechnaill proceeded to expand his power by war and diplomacy.

[11] On his death in 862, Máel Sechnaill's obituary titled him "King of all Ireland" (Old Irish: rí hÉrenn uile).

[12] On Máel Sechnaill's death, the Uí Néill kingship passed back to the northern branch, represented by Áed Findliath, son of Niall Caille.

Áed began his reign by marrying Máel Sechnaill's widow, Flann's mother, Land (d. 890), daughter of Dúngal mac Cerbaill, king of Osraige.

The historical records indicate that six times during his reign, or one year in three, the great Fair of Tailtiu was not held, "although there was no just and worthy reason for this".

That same year, the Annals of Ulster note that "Sigfrith son of Ímar, king of the Norsemen, was deceitfully killed by his kinsman".

[22] In 901, Flann's son Máel Ruanaid, described as "heir designate of Ireland", was killed, probably burnt in a hall along with other notables, by the Luigni of Connaught.

Flann did not long survive, dying near Mullingar, County Westmeath, according to the Prophecy of Berchán, on 25 May 916, after a reign of 36 years, 6 months, and 5 days.

His marriage to Gormlaith ingen Flann mac Conaing, daughter of the King of Brega, a key ally of his stepfather, was probably the first.

On Cormac's death in battle in 908, fighting against her father, she was married to Cerball mac Muirecáin of the Uí Dúnlainge, who supposedly abused her.

Eithne was also married to Flannácan, King of Brega, by whom she had a son named Máel Mithig, although whether this preceded her marriage to Flann is unclear.

[27] The alternating succession of the northern and southern Uí Néill to the kingship of Tara would finally break down in the time of Brian Boru.

The evident lack of filial loyalty among Flann's sons, Donnchad Donn being twice in rebellion against his father, may have prevented any such plans from coming to fruition.

[33] Alex Woolf suggested that Flann had not only attempted to monopolise the succession within his family, but had come close to instituting a national kingship in Ireland comparable to that created by his contemporaries Alfred the Great and Edward the Elder in England from their Kingdom of Wessex.

[34] Later Clann Cholmáin kings were descended from Flann, as was Congalach Cnogba, whose official pedigree pronounced him to be a member of the Síl nÁedo Sláine, the first of that branch of the Uí Néill to become King of Tara in two centuries, and whose last agnatic ancestor to have ruled from Tara was the eponymous Áed Sláine, ten generations before.

As Máire Herbert notes, Máel Mura depicts Tuathal as a 9th-century ruler, taking hostages from lesser kings, compelling their obedience and founding his kingship over Ireland on force.

The high king in Flann for Érinn has authority over the fir Érenn (the men of Ireland) and leads them in war.