Flann Sinna (English: Flann of the Shannon) (847 or 848 – 25 May 916) is the son of Máel Sechnaill mac Máele Ruanaid of Clann Cholmáin, a branch of the southern Uí Néill.

He is King of Mide (Meath) from 877 onwards and is counted as a High King of Ireland.

His mother Lann was a sister of Cerball mac Dúnlainge the king of Osraige.

Flann is chosen as the High King of Ireland, also known as King of Tara, following the death of his first cousin and stepfather Áed Findliath on November 20, 879.

Flann's reign follows the usual pattern of Irish high-kings, beginning by levying hostages and tribute from Leinster, and then to wars with Munster, Ulster and Connacht.

Flann is more successful than most, but 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, which 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 are thwarted when his favorite son Óengus is killed by his son-in-law and eventual successor Niall Glúndub, son of Áed Findliath, on 7 February 915.

Flann's other sons raise revolts and his authority collapses.