Animation Age Ghetto: Somewhat. It's recommended ages 5 to 7 on Netflix, despite the whole "vikings attacking and murdering an entire village" thing.
Anticlimax Boss: Crom Cruach. Despite being an ancient Celtic god who destroyed Aisling's entire family (and possibly species), it gets about a minute of screen time and is promptly defeated by a piece of chalk.
It's Short, So It Sucks: Perhaps the most common complaint with the film. The movie features a rich story and world with monks, a wall, a sacred book, fairies, Vikings, and an Eldritch Abomination, but packs all of it into 75 minutes of screen time.
Jerkass Woobie: Abbot Cellach post-attack, after he's been left broken by his guilt of not being able to protect his people and Brendan.
Mondegreen: What Brendan prays on the stone in the forest. The subtitles say "Oh no, it's hard enough," one can hear it as "I know eternal love," and the DVD Commentary mentions that it's a prayer in Irish, which doesn't make sense with the English subtitles...
It's the opening line of the Lord's Prayer in Irish: "Ár nAthair atá ar neamh...". The subtitles on the UK DVD just say [Prays in Irish].
Aisling is shown to be frightened of the monastery's cross in the courtyard and is unable to enter the tower where Brendan is. Fitting, since she's a fairy and her people are slowly being pushed out and forgotten as Christianity spreads across Ireland. Fairies in Irish folklore were also unable to enter buildings unless specifically invited, thus the use of Pangur Ban to free Brendan from the basement.
And, as mentioned above, the lavish detail the animators paid to the Book of Kells is gorgeous, and looks like it could easily be an excerpt from one of the pages.
Strawman Has a Point: Abbot Cellach's obsession with building his wall over preserving the books and his decision to forbid Brendan to go to the forest are portrayed as well-intentioned but ultimately misguided. And there's no doubt that he's not the best at expressing his affection towards Brendan. However, on the other hand, when Brendan does go into the forest, he's attacked by wolves and would have died if the local representant of the Fair Folk didn't come to help him. So the abbot was perfectly justified. Cellach's decision to protect the present (with his wall) instead of the future (the books and their knowledge) can seem justified in a time of invasions. And it's worth mentioning that, had he not lived in a world where Northmen are apparently unstoppable surhuman monsters, an invading army would have thought twice before attacking a heavily fortified location.