Ivar was a viking leader who raided Ireland in the mid-ninth century; he was reasonably probably the same man as the Ivar who was one of the leaders of the Great Heathen Army that conquered much of England in the 860s, and the prototype for Ivar the Boneless of the sagas and modern popular culture. In either case his descendants ended up ruling various bits of the British Isles in the following centuries, most notably Dublin, York, and Man and the Hebrides (the "Kingdom of the Isles").

Downham tries to piece together the career of this dynasty down to 1014, from the often laconic and cryptic mentions in various annals. The end date is that of the battle of Clontarf, and seems chosen more to keep her PhD thesis, on which this book is based, to a manageable size than because it marks a natural endpoint; the defeat of the Dubliners at the battle marked a step in the dynasty's decline, but certainly not its end. Ivar's descendants continued to rule Dublin itself long afterwards, and even longer in the Isles.

I found it all quite interesting, but it's also heavy going, with way too many characters sharing the same names, and far more may-have-beens than definite conclusions.