The Book of Kells is a "portal story"; one which involves some sort of gate providing access to another world. If the other world turns out to be a different planet, then the portal mechanism is usually wrapped up in pseudo-scientific blarney and the stories are classified as science fiction. The same might also be true of portals opening into parallel universes, or those which enable travel through time. However, if the author doesn't bother with such explanations but instead presents the portal as magical, the story is regarded as fantasy.
This is the case with The Book of Kells, in which a way is opened from present-day Ireland to 1,000 years in the past by an artist tracing the complex pattern on an ancient monument while listening to appropriate music. The irony is that this story is otherwise far more grounded in fact than most science fiction. Were it not for one brief episode concerning the apparent appearance of a goddess (or possibly a saint, depending on your perspective) and a miraculous healing which followed, the story set in ancient Ireland could well be classified as historical fiction. Ms MacAvoy is clearly an enthusiastic student of Celtic history, as indicated by the list of dedications to academics and other experts, and it shows: tenth century Ireland is portrayed in all its earthy realism. The tensions, scheming and outright warfare between the Irish and the Norsemen (somewhat uneasily settled in Dublin) with their differing cultures, attitudes and practices, are richly evoked. In fact, the frank descriptions of nudity and sex may be considered too rich for children (by their parents, if not by the children…).
What of the plot? Having accidentally opened the portal, the artist and his sometime girlfriend pass through and spend nearly all of the rest of the story in ancient Ireland, trying to adapt and survive while being pursued by vengeful Vikings. That's about it, really, apart from the ending which, while not unexpected, has a satisfying twist to it. However, the enjoyment of this book isn't down to the plot so much as the richness of the portrayal of ancient Ireland plus the quality of the writing, which is unusually intelligent and perceptive. The characters are distinct, credible, well-developed and sympathetically portrayed, and a recurrent stream of humour bubbles through the tale. Although it has moments of grimness, it's the kind of story you finish with a smile on your face, wishing for more.
The Book of Kells was first published in 1985, and I read it shortly afterwards. I remember enjoying it a lot at the time, and I did again. I have another, equally good pair of books by the same author: the highly original Tea With Black Dragon and its sequel, Twisting the Rope, which are on my re-reading list. I notice from her Wiki entry that she has also written two trilogies, the Damiano and Lens of the World series, plus a few other stand-alone works, but I haven't read these. I must try to get hold of them sometime; this is an author of rare and engaging quality.