The book goes through 5th century Ireland to 20th century New York, it's pretty imaginative and have very good descriptions of both times, truly magical really, will let you wanting to go to Ireland right away.
As with any time traveling romance the beginning it's a little slow because we need to get into the story who they are, what's going on and why, but after al this is laid out in the firsts chapters it's pretty nice and coming along the plot.
In this regards my only complaint it's that it seemed our protagonist was made a little unaware of things like come on she is an archeologist you describe her like very intelligent and then she misses all the critical points? seems unlikely not that it's impossible but that part I didn't love it.
The building of relationships didn't seemed rushed that was a good thing, you have time to enjoy how the tension keeps rising and the dynamic changing between all of them.
It does touch the religions points between Celtic and Christianity clashing together in Ireland, on early Christianity it was very hard to be a Christ follower really it seems no one really wanted it, but it did become a thing afterwards so the struggles between religions are interesting.
By the end I was very confused usually this type of books are self conclusive but this one didn't really reach any conclusion I do know there's a second book but it was a little weird that path, to see what it will get us later on.
The end was intriguing but since I didn't feel anything was solved it felt like something was missing of the book but not bad really just I will advice to read them in a row so you can get all the picture.