Why didn't Aisling just go into the tower herself instead of sending a spelled Pangur Ban? Even though it's been established she can move quickly and quietly, possibly without being seen? Because it's part of a church. Older stories have it that the Fair Folk can't enter religious buildings. It would also explain why she looks so nervous within the Abbey's courtyard.
Even better: she's one of the Tuatha Dé Danann, a semi-divine pre-Christian nature spirit, whose people were once worshiped as gods before the coming of the Christ. If your people were driven out of the minds and lands of their erstwhile subjects by the Church, you'd be afraid of them too. Note her terrified glance toward the cross in the courtyard.
Less specifically, trespassing through threshold law is a huge no-no for creatures like the Tuatha Dé, but catsgo whereverthey want.
Suddenly the line "you must go where I cannot" makes more sense...
Both Aidan and Aisling (in Aisling's Song) refer to the world as old and misty. The most common connotations arise — intangible, mysterious, haunting... but it just doesn't fit, because they are spoken as wise words when the harsh reality of the world has just made its presence known hard and sure. But this is exactly when it is misty — as in obscuring, as in hiding, between you and where you need to be, as in you cannot see but for the mist. Just as mist obscures one's view, the Abbot, so lost in his building of a wall that can't hold back the Norsemen, he cannot see the beauty in his drawings of that very wall. If you are commanded to never leave, you will never see the Forest, know only the prison and you cannot sing for release, or understand that a book be more than sheaves of parchment with ink marks upon them.
As shown in Folio 7v◊ green is actually one of the most common colors (along with purple, lilac, red, pink and yellow according to The Other Wiki) used in the real Book of Kells. Where truth differs is that the green is made from copper (verdgris), not berries. Of course that would be a bit tricky to fit into the plot.
Oak galls- the 'berries' Brendan picked from the oak tree- were actually used to make BLACK ink, but it's easy to understand why they tweaked it so it would be green. Firstly, because black was already associated in the film with Crom Cruach and the marauding Norseman; secondly, green works better thematically, as it represents nature generally and Ireland specifically. Call it artistic license.
Genius Bonus: They could have called this Genius Bonus: The Movie.
Pangur Bán is named after the oldest surviving poem in the Irish language. Written by... a young monk in the margins of his study, about his pet cat. An excerpt from the poem itself is recited over the credits.
"Aisling" (pronounced Ash-ling) is an Irish girl's name meaning "dream" or "vision", but it's also the name of a genre of Irish poetry. In these poems, a woman appears as the Anthropomorphic Personification of Ireland and speaks about the country's troubles, followed by a prediction of a better future. The writers for the movie decided to play with the concept by making the female figure a mischievous little girl instead of a serious older woman.
Aisling's opening monologue is based on another very old Irish poem called "The Song of Tuan Mac Cairill", one of the Tuatha De Danann who survived among humans by taking on the forms of a salmon, a deer and a wolf, rather like we see Aisling doing. Seems like there's a lot of references to Irish poetry here...
The Abbot (who's based on Cellach, abbot of Iona who has fled to Kells in 814 due to viking invasions) symbolizes the newer Roman Christianity◊ that took hold in Ireland about a thousand years ago, in contrast to early Celtic Christian attitudes which were allowed to develop during Roman decline, a more tolerant, generous, and less rigid form than the prevailing Roman Christianity.
Except that there's no real evidence that there ever was a "Celtic Christianity" or that it was either more or less tolerant than the Roman church.
I'll have to disagree with that. Syncretic Christianity? Passing Penance? Material focuses from Columbanus? Perhaps you mean that there was no unified opposition-strand of Christianity that could stretch over all of Western Europe, or even just the Celtic/Brythonic world. However, there were huge differences to those churches more sublimate to the Papacy, especially in the insular Isle. Given the high number of heresies that flourished until realignments occurred (if memory serves, it was the Synod of Rathbreasail in Ireland, with exceptions in Munster), it would appear that it was far more tolerant - or less aligned with, or simply less focused on assimilation of pre-Christian beliefs - than Roman Catholicism.
Aidan is named after St. Aidan of Lindisfarne (died 651), a missionary from the island of Iona who was instrumental in spreading the Celtic branch of Christianity in northern England.
The book's cover actually was stolen and has long since been lost, so the animators designed a plausible-looking cover from historical descriptions they pieced together. (And painstakingly traced the Chi Rho page and fully restored it). Whoa.
The battle with Crom Cruach might be a reference to the way Italian medieval painter Giotto demonstrated his skill to the Pope. If so, that scene is essentially Brendan's rite of passage as an artist.
The map of Kells is also map of the world: it bears strong resemblance to the schematic T-O map, with the tower in the middle representing Jerusalem and appropriately pointing toward the heaven.
Crom Cruach takes the form of a huge snake. It is said that St. Patrick was the one who ended worship of Crom Cruach in Ireland. What else was St. Patrick said to have done? Banish all snakes from Ireland!
Brendan tells Aisling that Crom Cruach is all Pagan nonsense. Aisling is a fey so she is Pagan nonsense too. Of COURSE she's terrified of Crom!
She's also incredibly nervous when in the city itself, glancing fearfully at a cross... Aisling is a fey, "Pagan nonsense," if you will, the city is a Christian one, the cross represents that, and she cannot enter the buildings. In folklore, the fair folk are forbidden from entering a holy place, and as for her fear... well, considering the severity in how Christianity tried to squash Pagan concepts, of course she's terrified!
How come Brendan is so easily capable of accepting the existence of Aisling the fairy, but not Crom Cruach? Being a Christian monk, the existence of something like a "nature spirit" is simply easier to fit within his worldview. A fairy could be a part of the world, perhaps somehow related to angels or simply another type of being. Crom Cruach, however, is supposed to be another god, something completely and utterly antithetical to all the theology Brendan has ever learned.
Aisling helping Brendan enter Crom Cruach's lair so he can use his Eye to make the book was a massive leap of faith on her part, and not just in Brendan. She was putting faith in a 'book' - something she had never heard of before - and in a religion she'd also never heard of, one attempting to replace worship of Crom Cruach, which had in turn replaced belief in her own kind (metaphorized by Crom killing her people). This is what she hopes for. This is why she begs Brendan to turn the darkness into...light.
When the Vikings attacked Kells, the Abbot believed that they killed Brendan. Brendan was stuck in a building with Brother Aidan. And the Abbot had locked him in there. Considering this fact, and knowing that the Abbot spent years believing Brendan was dead, you realize that the poor man had to live for years under the belief that he indirectly killed his own nephew. No wonder he's so broken in the ending...
Not just a few, either —according to the design drawings, older!Brendan was in his thirties. 'Nearly twenty years of being broken with grief, thinking he was what killed his twelve year old nephew....'
It gets worse when you read the prequel comic, and realize the Abbot fought his way through a Northmen attack to save Brendan as a baby-and lost his sister, his brother-in-law, and their ENTIRE village to the same attack. Seeing the Abbot's determination to save baby Brendan drives home even more how much he loves him and how he really would do anything to protect him... and how very devastating it was when ultimately he believed he was responsible for Brendan's death.
The Abbot's wall was designed to keep the Vikings out. Instead, once they broke through, it trapped the innocent people of Kells inside and made them easy targets. Without the wall, maybe some of them could have escaped to the forest. Brendan and Aidan only escape because Brendan knows a secret passageway to the outside.