Flannán is the subject of a late and rather fabulous Life composed at Killaloe in the 1160s at the instance of Domnall Mór Ua Briain (qv), and apparently based on an earlier fragmentary vita. The Life credits Flannán with such miraculous powers that he prayed to be deformed lest he become proud. Nonetheless, he is portrayed as a man of gentle birth and well educated in letters who cultivated humility by engaging in agricultural work and domestic chores. It is claimed that Flannán came to Killaloe to live under the rule of Molua, but personal contact between the two seems unlikely, as Molua is assigned to the sixth century. While the inclusion of Flannán in the tract ‘The unity of Máel-ruain’ need not necessarily imply personal association with Máel-ruain (qv) of Tallaght, it suggests that he had a role in the Céli Dé monastic reform movement of the eighth century.
It has been suggested that Flannán should be identified with the cleric of that name from Cell Aird (Killard, barony of Ibrickan, Co. Clare), who died in 778. The connection of Flannán with Killaloe, where there is an oratory dedicated to him, may indeed have been posthumous, but was certainly promoted by Ua Briain interests. Flannán's cult spread as the power of the Dál Cais kings increased. It became established in Ossory, where Flannán was alleged to have visited the saintly Broccán at Cluain Immorchair, and in the Hebrides – reflecting, perhaps, Ua Briain lordship of Man and the Isles.
Though the year of his death is uncertain, Flannán son of Tairdelbach – sometimes styled ‘confessor’ – probably died on 18 December, under which date he is commemorated in the martyrologies. His relics were preserved at Killaloe for many years; his power was believed to have caused the downfall of the Connacht king Áed Ua Conchobair (qv), who burned the settlement in 1061. Stories concerning Flannán, some featuring motifs borrowed from kingship tradition, survive in modern folklore.