I would really like to give this a higher rating, but I just can't do better than 3½ stars, which isn't allowed here.
Overall, this is a good retelling of the beginning of the Norman invasion of Ireland. Butler gets all of the main points - events and people - correct. I would expect that of someone of Irish heritage. I am also of Irish heritage - the O'Brynes of County Wicklow - and I knew a lot of the outline, but not a lot of the details. Research on-line gave me reason to believe that the author had done his own research well.
But that's as far as it goes. The writing style is clumsy and awkward. This might well be as much a function of weak editing by someone not very familiar with Irish history, or it might have been because of some really bad transfer from print editions to the e-book which I read. Either way, it was difficult to follow a lot of the action due to sudden shifts of viewpoint within chapters. This was really quite bizarre, because it has been common practice in fiction typography, for over a century, to put bars of some sort - a long line, either simple or ornate, a row of asterisks, a small box with a dash on either side, anything of that sort - between one section in a chapter and another. At the least, a gap of several blank lines will work. There is nothing like this anywhere in this novel. Had some of the chapters been shorter, it might not have been so bad.
Another, more serious, problem was Butler's insistence on using full Irish names, without any effort made to explain how to pronounce them. I know a small bit of Irish Gaelic, enough to know that Phádraig is Patrick, Dubhlinn is Dublin, Diarmaid is Dermit/Dirmat, that Ui became O', and a few other associations. I could struggle along with most of the rest, but as so many English-speakers do with Russian novels, I had to "blip" over most of it. A simple listing of the Dramatis Personae and place names, and how they transliterate in English, would have been nice, and not terribly difficult, nor very long. Certainly the author hopes that the inquisitive reader would do some individual research, but a little help from him to get things moving would have been very nice.
Bottom line: I enjoyed the narrative and its cleaving fairly closely to actual history, at least as we understand it nearly a millennium later. I was able to surmount the difficulties that I outline, above. However, I have little interest in reading any further novels in this set: I don't have enough time left on this planet to keep struggling with these issues, especially as I already have plenty other books to read.