Short answers, pulled from the story.
Cogad Gáedel re Gallaib, meaning The War of the Irish with the Foreigners, is a medieval Irish chronicle about the Viking raids in Ireland and Brian Boru's war against the Vikings and the Uí Ímair dynasty. It covers events from the Battle of Sulcoit in 967 to Brian Boru's death at the Battle of Clontarf in 1014.
Cogad Gáedel re Gallaib was written in the early twelfth century, with scholars suggesting composition sometime between 1103 and 1111 based on internal evidence and allusions to Muirchertach Ua Briain, who died in 1119. This means it was composed at least a hundred years after the events it describes.
Cogad Gáedel re Gallaib survives in three manuscripts: the Book of Leinster from around 1160, a Dublin Manuscript from the fourteenth century, and a Brussels Manuscript from 1635.
Cogad Gáedel re Gallaib was written to glorify Brian Boru and argue that his Dál gCais dynasty deserved Ireland's high kingship. It was composed during the rule of Muirchertach Ua Briain, when the Ua Briain sept was struggling to retain the high kingship, and was intended to draw a parallel between Muirchertach and his ancestor Brian Boru.
Modern scholars describe Cogad Gáedel re Gallaib as brilliant propaganda written in a bombastic style full of patriotic hyperbole. Although its accuracy is uncertain, it remains a valuable source of information about the Viking Age in Ireland.
The scholar Einar Olafur Sveinsson proposed in 1954 that Njáls saga, composed around 1280, incorporated material from a lost thirteenth-century Icelandic text called Brjáns saga. The relationship between these texts and the Cogad remains a matter of debate, as all Icelandic written sources are considerably later than the Irish chronicle.