King Sweeney, also known as "Mad King Sweeney," was a legendary king of Ulster in Ireland whose story is told in Buile Suibhne, an Irish poem mixed with prose which exists in manuscripts dating from 1671 - 1674 but which was almost surely written and circulated in its modern form sometime in the thirteenth to fifteenth centuries. It is likely, from references in works going back to the tenth century, that some form of the tale of the mad king goes back to the first millennium.

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 was briefly restored by his kinsmen, who very gently coaxed him back to earth, but, while recuperating, a mill hag taunted him into a contest of leaping. As Sweeney leapt along after the hag, he again took flight and returned to madness. Eventually, after travels throughout Ireland and Western England, Sweeney was harbored by Bishop Moling. He lived, broken and old, with the bishop, and the bishop entrusted his care to a parish woman. Unfortunately, that woman's husband, a herder, grew jealous and killed Sweeney with a spear. On his death, Sweeney received the sacrament and died in reconciliation.

The poetry in the story of Sweeney is exceptional, and the story itself of the mad and exiled king who composes verse as he travels has held the imagination of poets through to 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, while the author Flann O'Brien worked a translation of Buile Suibhne into his comic novel At Swim-Two-Birds.