In the legend, Bishop Ronan Finn set up a church in Sweeney's kingdom, and the king grew enraged by the sound of his bell, laid hands on the bishop, threw his psalter into a lake, and meant to kill the bishop. Sweeney then went to engage in battle, in the Battle of Moira (637 A.D.). Prior to the battle, Bishop Ronan blessed the troops. Sweeney took this action as a taunt and killed one of the bishop's psalmists with a spear and threw another spear at Ronan himself. The spear struck Ronan's bell and broke it. At this, Ronan cursed Sweeney with madness. His curse was: 1) that as the sound of the bell had been broken, so now would any sharp sound send Sweeney into madness, 2) as Sweeney had killed one of Ronan's monks, so would Sweeney die at spear point. When the battle began, Sweeney went insane. His weapons dropped, and he began to levitate like a bird.
From that point on, Sweeney leapt from spot to spot, like a bird. Also like a bird, he could never trust humans. His kinsmen and subjects sent him mad with fear, and he could only flee from place to place, living naked and hungry. After seven years in the wild, Sweeney's reason is briefly restored by his kinsmen, but a mill hag taunts him into a contest of leaping and provokes him back to his madness and avian life. Eventually, after travels throughout Ireland and Western England, Sweeney was harbored by Bishop Moling. Unfortunately, a jealous herder slayed Sweeney because his wife had been giving Sweeney milk. On his death, Sweeney receives the sacrament and dies in reconciliation.
The poetry in the story of Sweeney is exceptional, and the story itself of the mad and exiled artist has held the imagination of poets through the twentieth century. At every stop in his flight, Sweeney pauses to give a poem on the location and his plight, and his descriptions of the countryside and nature, as well as his pathos, are affecting. Many poets have invoked Sweeney -- most notably T. S. Eliot and Seamus Heaney.