The poem is about an Irish warrior chieftain named Sweeny. Sweeny, annoyed by the pious activities and ringing bell of a cleric, throws the cleric's missal in a lake. After further altercation the priest curses Sweeny who is transformed, or goes mad and thinks he is tranformed, into a bird. He leaves his beautiful wife and castle to journey naked in the wilds of Ireland, living on watercress, hazelnuts, berries and sleeping in trees. His suffering is intense but he often breaks into rapturous visionary lyricism for the wilderness of his native Island, naming the sacred Celtic trees and mountains, glens, coves and rivers. He dies mad and alone, the victim of misplaced jealousy.
I see the poem in 3 ways: first as a movingly subversive means by which Irish poets kept alive the Celtic vision of the land while allowing this vision to survive in a repressive Christian culture which sought to bury Celtic lore and history; second as a probing reversal of the Adam and Eve story in which God's curse removes them from nature, but sends Sweeny into nature; and third as a description of the damage of war on the soul, Sweeny's affliction bears much resemblance to patterns of post traumatic stress.
The beauty Sweeny sees is all the more piercing because of his hardship. His banishment from humanity wildly offset by the fact that he becomes a creature of flight, a bird of the air, unbound from gravity.