The dark witch thing isn't explained very well. Witch is the term they use, while the "evil" one is a sorcerer. Dark tend to have negative connotations (dark=evil), so a little explanation would have been nice. It doesn't take long to realize that dark doesn't mean evil in this series. The term difference becomes clear once Iona learns that Fin is a descendant of the sorcerer they are trying to defeat and it's made clear that he's a witch and on their side.
I expected a little more world building in this one, I know it's magical realism but some info would be nice. Iona is the only one that hasn't grown up learning magic and she grew up in America and not Ireland. I really liked Iona's connection to horses and I'm hoping for the same sort of connection with Conner and his hawk and Branna and her hound.
I was a little worried that this series was going to be like Robert's Circle Trilogy. They are both magical realism with three people working to defeat an evil. That series includes time travel, which was the big issue I had with it. This book took a while to get into, since the first three chapters are from Sorcha's (the first Dark Witch) point of view, back in the 1200's. I wasn't all that surprised at first, but three chapters in the past confused me quite a bit (normally something like that is in the prologue).
I could have done without Boyle's stupid moment were he suggests that his affection for Iona is due to a spell. It gets brushed off, mostly. She overhears and gets mad (can't blame her), they break up. Then he claims he'd just been upset and hadn't really meant it. BS! Yeah, he was mad but he wouldn't have said it if he hadn't at least thought it at some point. They become friends but get back together without ever discussing it.