The Secret of Kells/WMG

Pangur Ban is also a fairy of some sort.
It seems possible to me, she could just be Mode Locked like Aisling. She's also way too smart to just be a normal cat, there has to be some magic there.
 * Would explain why she seems to be so long-lived.
 * She is also white/silver-furred and has Heterochromia iridium (two differently coloured eyes; a symbol for paranormal abilities). Not only that, but one eye has Brendan's eye colour (blue), and her other Aisling's (green).
 * She's definitely not like Aisling. She was able to enter the church to get the key. Aisling was not.

Crom Cruach was exorcised by St. Patrick.
St. Patrick drove the snakes out of Ireland and Crom Cruach is a giant one.
 * Actually, according to legend, St. Patrick was the one who exorcised Crom Cruach, except Crom took the form of a demon instead of a snake.

The book is evil
Everything bad that happens can be attributed at least somewhat to the book. Brenden risks his life multiple times for the book, at least one of which was completely unnecessary (they could have sent someone armed who knew their way around for the berries). The Abbot, who's barely seen the book, does everything he can to protect Brenden. Aidan, who has written much of the book, couldn't seem to care less. Iona, where the book had been for the most time, never did much in the way of defenses. Kells had walls, but the door wasn't strong enough to hold the Vikings out. Possibly because people were working on the book, rather than on strengthening the walls, like the Abbot wanted to do. After Brendan gets the replacement crystal, he never sees Aisling again.

And of course,. I'd consider that a bad sign.
 * the Book of Kells is a Bible. Not sure if that makes it seem more evil, but...yeah, It's a work of art/bible..
 * Not to mention he actually did see Aisling again.
 * He saw her human form once, a decade or so later, for less than a second. He saw her wolf form a few times.

The Eye of Crom Cruach bestows wisdom.
Brendan certainly seems smarter after he gets it.
 * Considering he had to pick a fight with Crom Crauch to get it, he couldn't have easily acted stupider after.

Aisling and Brendan met before the events of the movie
In the comic prologue that reveals Brendan and abbot Cellach's back stories. When Cellach flees from the vikings while carrying Brendan, there is a scene where he jumps into the water. in this scene baby Brendan is looking at a white fish with green eyes. since Aisling has the ability to change form and she said at some point that she was a fish or something there's no reason why the fish can't be her.