Beyond Selidor

The whole book is amazing from start to finish. You definitely don't need to be familiar with medieval Irish literature to make sense of the book — it's a retelling of one story, and makes reference to a lot of others, but it's always given context. I think the only thing that familiarity gives you (for context: I have a PhD in medieval Irish literature) is that you notice which other stories are being referred to before the point that they get fully explained. But that's really not necessary to get something out of the book.

I should warn you that it's not a happy story, though. It's not a comfortable thing to read.