It describes the fictional history of the Déisi, a group that had gained political power in parts of Ireland during the Early Middle Ages.
Part of the text's purpose is to provide the kings of the Déisi – historically the descendants of unlanded vassals to other tribes – with a mythical noble origin as the heirs to a dynasty expelled from Tara.
The story survives in two different Old Irish versions, which contain essentially the same narrative supplemented by singular additions, including tangential episodes, lists of names, and poetry.
[5] The B manuscripts all contain some reference to the injury visited upon Cormac mac Airt, one of the characters: the Book of the Dun Cow has Tucait innarba na n'Dési im Mumain & aided Cormaic, H. 3.
[4] Marie Henri d'Arbois de Jubainville's Catalogue further lists two possibly related titles: Longes Eithne Uathaige and Tochomlad na nDési a Temraig.
Because the law forbids a high king to have a physical blemish, Cormac must retire to a ráth outside Tara, and defer the kingship to his son Cairbre Lifechair.
[10] Eventually the Uí Bairrche warrior Eochu Guinech ejects the Déisi and they are compelled to move further south into Ard Ladrann, thereafter known as the "Land of the Wandering Host".
Section 8 tells the story of Ethne the Dread, daughter of then King of Leinster Crimthann mac Énnai (d. 483) and an eventual fosterling of the Déisi.
At her birth a druid prophesies that under Ethne her mother's kindred will seize what will be their homeland; hearing this her parents feed her the flesh of young boys to accelerate her growth.
They receive some relief when Óengus mac Nad Froích (d. 489), King of Munster, proposes to marry Ethne the Dread, who has been traveling with the Déisi.
With the help of Lugaid Laigde Cosc, the seer-judge of the Corcu Loígde and Cashel, they devise a trick: learning of a prophecy that whichever side draws first blood the next day will be routed, the Déisi druids transform one of their men into a red cow.
[13] It likewise begins with the kidnapping of Forach by Cormac's son (here Cellach), but in this version Óengus is initially too busy avenging family insults with his famous spear to save her.
The story continues with the Déisis' expulsion from Tara, their sojourn in Leinster, and their patriation in Munster following the marriage arrangement between Ethne the Dread and Óengus mac Nad Froích.
[14] Of the material not in the A version, by far the most substantial is in a lengthy tangential episode dealing with the birth and childhood of Óengus's foster son Corc Duibne, progenitor of the Corcu Duibne, whose name is merely mentioned in A. Corc and his brother are born of incest to Coirpre Múse and Duihind, children of the king of Munster Conaire Cóem, and their nativity brings famine to the land.
The druid's wife, Boí, performs purification rituals that ultimately transfer Corc's curse to an otherworldy white cow, allowing him to return to his family.
[16] Section 11 of A describes Eochaid son of Artchorp traveling oversea with his descendants and settling in Demed (Demetia, later known as Dyfed in Wales).
From Eochaid and his descendants springs the line of kings of Dyfed, whose descent is given down to "Tualodor mac Rígin" (Tudor map Regin).
Eoin MacNeill, who treats the literary accounts as largely historical, suggests that a Déisi settlement in Dyfed could have existed as a subordinate unit to a larger Uí Liatháin-led colony until the latter was expelled under Cunedda.