This is serious, detailed history about a neglected aspect of UK / Irish history. I've read a lot of WW2 history and Irish history, but the majority of this book was new to me.
As other reviewers have noted, none of the main protagonists come out of the book unscathed. To simplify, de Valera was frequently devious and obsessive, Churchill bombastic, the Northern Unionists manipulative and the Germans incredibly naive and ill-informed.
De Valera's government certainly made their mistakes, especially the crass decision to pay a visit to the German embassy to pay their respects after Hitler's death, but was their strategy wrong? The Irish state in 1945 was relatively unscathed, even if without Northern Ireland. If they had joined the Allies, would a united Ireland have resulted after an Allied victory? I'm not convinced, and there was a very high probability that doing this would have started something near another civil war.
One irony of this period is that the agreement to hand back the Treaty Ports probably strengthened the hand of the northern Unionists by increasing Britain's dependence on their ports.The Unionists exploited this to the hilt both during and after the war, projecting a picture of "Loyal Ulster" that really doesn't ring true - their contribution to the was effort was very poor, with the exception of providing the ports (which they didn't have much choice about.) Another irony is that the northern Unionists gained so much politically (e.g. the late 40s declaration the the British government that there would be no change without the consent of the majority of Northern Ireland's population) despite its government during the war being staggeringly incompetent, especially in dealing with German air raids.
This book is very relevant to post-war Irish history, even up to today.