Fidelma is a little too smart and her companion (to whom everything needs to be explained for the benefit of the reader), Eadulf, a little too stupid - except when he needs to be really, really smart for the plot. So what we have here is Ms. Sherlock and Brother Watson of 7th century Ireland. It was a bit daunting to start with several pages to put the reader up on history and Irish religion and society, plus a cast of characters (with names that were hard for me to follow), and a map. As it went on, though, it was only the names that still threw me. To remember Eadulf, I had to think of Hitler and make the spelling go wonky. Sechnassuch looks so much like sassenach to me that I stumble over it each time I see it. Maybe some pronunciation hints would be helpful as well. Fidelma is about the easiest one to remember.
There's also a lot of Latin, "cleverly" translated usually by the person who heard it, and a lot of Old Irish (that has a long name that I've abandoned for the crossword puzzle word, Erse). Despite all the work one must do reading, it is a satisfying mystery. And if Fidelma calls everyone into the room for the Big Explanation, she can be forgiven because she's a dálaigh and she's pretty much conducting a court case.
Although not required to be celibate at this point in history and especially in Ireland, if there is any sexual tension between Fidelma and Eadulf, it's not showing, although Fidelma's brother suspects and there have been at least 6 books before this, so who knows what's going on. Eadulf, as his Germanic name suggests, is a Saxon but has lived and studied in Ireland for quite a while. He is embarrassed by the attentions of the 14 year old Esnad, who is hot to trot. His reluctance is based on her age and probably a dislike of her forwardness and Fidelma seems amused. There seems to be plenty of sex going on around them - and slapping when unwanted advances are made - but nothing going on between Fidelma and Eadulf. Also, all the sex seems to be happening "offstage" so only descriptions of murder scenes are icky.
Fidelma's brother, Colgu, sends her to negotiate with an old religion clan to build a Christian church in their area. On the way, Fidelma and Eadulf come across what appears to be a ritual slaying. At the clan's castle, there is an unexpected priest from another domain, and when he is killed, Fidelma is blamed. The whole negotiation thing is going downhill fast. Can Fidelma solve the mysteries if she's locked up? Heh!
Lots and lots of historic detail for those of us who love to wallow in that.