Brian Boru was the King of Munster in Ireland in the late 10th and early 11th centuries. Whereas his great rival King Sitric Silkbeard of Dublin was a Norseman, Boru was Irish-born and attempted to use this status to his political advantage. Following the first Leinster Revolt in 1000, he had succeeded in severely undermining Sitric's power. In 1006, Sitric lamented that he had become little more than a lapdog to Brian.
The Book of Kells was crucial to Brian's machinations. Although the book was written on the Scottish island of Iona in the 8th century, it was a powerful symbol of Irishness. Consequently, Sitric was concerned that if Brian gained possession of the book, he would be able to use it to gain control of Dublin and Ireland as a whole. In 1006, he sent one of his warriors, Olaf Eriksson, to enlist the help of Brother Bernard, the chief librarian at the Abbey of Kells, in protecting the book so as to prevent Brian from using it as a pawn to place himself on the throne of Dublin.
However, the Eighth Doctor and his companion Tamsin Drew almost entirely derailed Sitric's plot as they were labouring under the misapprehension that Brother Bernard was the Doctor's old enemy and fellow renegade Time Lord, the Monk. In actuality, Abbot Thelonious was the Monk, who sought the Book of Kells as he had instructed the monks at the Abbey to re-create a vital part of the directional unit of his TARDIS, which the First Doctor had stolen many years earlier in their respective personal timelines.
Although Sitric's plan to keep the book out of Brian's hands was successful, the King of Dublin's position was further destabilised by a second revolt in Leinster several years later and the Battle of Clontarf in 1014. (AUDIO: The Book of Kells)