Although the characters, fantasy elements, and plot specifics of this movie are fictional, there is a real "Book of Kells", an illuminated, heavily illustrated rendering of the Four Gospels of the Christian bible, that dates from the Early Medieval period (probably the early Eighth Century) in Ireland. The best historical and archaeological evidence suggests that, starting shortly after it was finished, the book was moved several times (including, as depicted in the movie, during an Viking invasion) and lost for various periods. It has been housed at the library of Trinity College, Dublin, since the Seventeenth Century, and is considered perhaps the single most valuable cultural artifact of Irish History that has ever been discovered. Some of the design concepts for the movie echo aspects of the original Book of Kells; for instance, the shapes of the repeated tree patterns as Brendan enters the forest are quite similar to the arrangement and shapes of the columns and arches in the "Eusebian Canons, Folio 5R" page of the real book.
Aisling, the fairy girl, is named after a 17th-century genre of Irish poetry. Aisling is Irish for "dream vision." In an Aisling poem, the poet would describe receiving a vision of the spirit of Ireland, who appeared to him in the form of a beautiful young woman. The female spirit of Ireland would inspire the poet to write about his homeland, and serve as his guide in creating the poem (much as Aisling serves as Brendan's guide in the film).
Pangur Ban is named for a cat who appears in an 8th Century Old Irish poem written by a monk to his pet cat. Aidan (Mick Lally) can be heard reciting a condensed rendition of the poem in modern Irish in the credits.
Development on the film began in 1999 when director Tomm Moore and some of his friends first saw a series of animated films: Richard Williams's The Thief and the Cobbler (1993), Disney's Mulan (1998) and Hayao Miyazaki films. The cultural style and influence of those films (respectively Persia, China and Japan) inspired Moore to make a similarly styled film based on Irish art/culture.
The oak "berries" Brendan collects are actually oak tree marble galls, formed by a gall wasp laying eggs in the tree, which are then used to make iron gall ink. Traces of iron gall ink have been found on the Dead Sea Scrolls and the "lost" Book of Judas.
Brendan is 12, and the prequel comic included in the Blu-Ray version indicates that he was rescued from the Vikings as an infant by his uncle Cellach in 790 AD. The movie therefore takes place mostly in 802 AD. This was the actual date of a real-life Viking raid on Iona, the event that drove Aidan and the book to Kells for safety.