Sitric Silkbeard was a Norseman who became King of Dublin in Ireland in 989. Following the first Leinster Revolt in 1000, however, his position was undermined by the political machinations of his great rival, the Irish-born King Brian Boru of Munster.
The Book of Kells was crucial to King Sitric's plan to prevent the outbreak of war in Ireland in 1006. Although the book was written on the Scottish island on Iona in the 8th century, it was a powerful symbol of Irishness. 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. 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 in his scheme to place himself on the throne of Dublin.
Sitric himself arrived at the Abbey after a three day ride on his horse Sleipnir, which was named after the eight-legged horse of the same name from Norse mythology, from Dublin. After the book was stolen, he told Abbot Thelonious that he willing to tear the Abbey apart and kill every monk within its walls if it meant finding it and therefore preventing war. 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.
After being partially burned by Tamsin and being lost for 80 days, the Book of Kells was recovered, in accordance with recorded history as chronicled in the Annals of Ulster, and returned to the Abbey. In the years that followed, Sitric's position was further destabilised by a second revolt in Leinster and the Battle of Clontarf in 1014. (BFA: The Book of Kells)