Another in the Tempus series of monographs (like Maria Kelly's Black Death) on Irish history. Lenihan takes the 1 July battle and examines it in depth from military, political and above all psychological perspectives. His unpacking of what James II, William III and Louis XIV were really up to is most enlightening - he doesn't believe James had aimed at much more than the restoration of Catholic rights before 1688, which chimes with my instinct, but then goes on to say that in 1690 William's hold on Britain was still far from complete, and the Irish campaign was necessary as much as anything to satisfy the Westminster Parliament.
Lenihan's analysis of the military styles of the kings on each side of the Boyne and their commanders is even more impressive: William and Schomberg were second-rate (and he gives examples from William's other battles to support this), but James and Lauzun were third-rate - the best evidence of this being that the battle took place at the Boyne at all, rather than the much more strategic Moyry Pass, abandoned by the Jacobites without a fight.
The description of the Boyne battle itself is a forensic dissection, with Lenihan slightly (and unnecessarily) apologetic for the amount of detail, honest about the gaps and inconsistencies in his sources, and also honest about the fact that the most decisive moment in the battle was something which didn't happen on the previous evening, when William was grazed on the shoulder by a cannon-ball; had he been killed at that stage, his forces would probably still have won the battle (if it went ahead) but certainly lost the war, or at least concluded it on much less favourable terms. But the fact that William, though wounded, carried the field, while James fled despite a surprisingly low number of casualties, was enough to set the mythology of the battle and the reputation of both men.
Having said up front that I really enjoyed the text, I am sorry to say that there are several aspects of its presentation which fall below the standards I would expect from a responsible publisher. The maps are too few, and are confusingly placed and labelled, which is something you really don't want in a book on military history. As with Kelly's book on the Black Death, the index has serious deficiencies. And James II's own memoirs - a key source!! - are confusingly cited; it is implied that they are reproduced in Clarke's 1816 biography, but it would have been nice to be clear. Despite all this I'd recommend the book unreservedly to anyone who already has a decent idea of the historical and geographical terrain.