Sweeney Astray is Seamus Heaney's translation of the medieval Irish text Buile Suibhne, ‘The Madness of Suibhne,’ a tale in verse and prose that describes how Suibhne (Sweeney), an Ulster king, clashes with a local cleric, Rónán, and having been cursed by the cleric, goes mad during the battle of Magh Rath (modernised to Moira in Heaney's version) in the year 637. Believing himself to have been transformed into a bird, Suibhne retreats into the wilderness where he lives a life of hardship, wandering from place to place, and reciting poetry that describes his suffering. Attempts by those close to him to help Suibhne recover his senses meet only temporary success, and he remains an outcast until his reconciliation with the Church through another cleric, Moling, which takes place as Suibhne lies dying.

Heaney's translation, published in 1983, is based upon J. G. O'Keeffe's edition and translation, published in 1913 by the Irish Texts Society. O'Keeffe's edition is based on a manuscript written in the 1670s, and there are no surviving manuscripts from before the seventeenth century. Heaney nonetheless cites O'Keeffe as believing on linguistic evidence that ‘the text might have been composed at any time between the years 1200 and 1500,’ notes external evidence to suggest that ‘the thing was already taking shape in the ninth century,’ and suggests that the work derives from earlier traditions yet, leading back towards the historical seventh-century battle described early in the narrative (p. v).