Historical ard rí [high king] of Ireland, c.545–c.565/8, son of Fergus Cerrbél, a leader of the southern Uí Néill, and reputedly the last pagan monarch. He was the last to celebrate the pagan ritual of feis Temrach [the feast of Tara], i.e. of ‘sleeping’ with the local earth-goddess. The annals record that Diarmait was defeated by the northern Uí Néill at the Battle of Cúl Dremne, ad 561. He is better remembered for bringing curses from holy men, one that would leave Tara desolate until Doomsday. A fanciful St Rúadán (sometimes St Rónán) cursed Tara. Trouble began when one of Diarmait's retainers was killed by Áed Guaire, related in fosterage to St Rúadán. When Diarmait sent armed men to seize Áed, Rúadán concealed him, and so the king had the saint arrested and tried in his place. For this outrage, condemned by other clergy as well, St Rúadán declared that Tara should remain desolate forever.
Other legends describe three prophecies of Diarmait's death, one by a St Rónán, another by a St Ciarán (not identified as being of either Clonmacnoise or Seir), and a third by the druid Bec mac Dé. Diarmait's troubles begin when he punishes Flann mac Díma for adultery with his wife, Mugain (3). Diarmait has Flann's fortress burned over his head, forcing him to seek refuge in a vat of water, where he drowns at Beltaine. For this, according to St Rónán, a roofbeam will fall on Diarmait's head. St Ciarán foretells that Diarmait will die as Flann had. But Diarmait's own druid, Bec mac Dé, prophesies a three fold death: by drowning, burning, and having a roofbeam fall on his head. Diarmait will be killed, the druid says, by Áed Dub [Ir., Dark Fire], Flann's kinsman, in the house of Banbán the hospitaller. It will happen the night he wears a shirt grown from a single flax seed, drinks ale brewed from one grain of corn, and eats pork from a sow that has never farrowed. As these events seem easy to avoid, Diarmait dismisses the prophecy, even when Banbán invites him to a banquet; his wife, Mugain, however, accepts the prophecy and refuses to attend.
Seeing Mugain absent, Banbán offers his own daughter to be Diarmait's bed-partner for the night. Upon receiving a nightshirt, meat, and ale, Diarmait is not suspicious. When the girl tells him that the nightshirt has been made from a single flax seed, the pork has come from a sow that has not farrowed, and the ale has been brewed from a single grain of corn, the high king knows that the prophecy has been fulfilled. Before Diarmait can escape, Áed Dub meets him at the door, piercing him with his spear. Fleeing to the back of the house, wounded, Diarmait finds himself engulfed in flames; Áed Dub's men have set the house ablaze. Hoping to escape the conflagration, Diarmait leaps into a vat of ale; and a flaming roofbeam falls on his head. Bec mac Dé's prophecy is fulfilled.