The 3 out of five rating is based on a few things:
Firstly, as explained in its introduction, almost all newer names are omitted. Their justification is fine, but at the same time they purport usage for modern naming purposes.
Secondly, the positioning of older forms first and modern versions second is in my opinion, entirely wrongheaded. It poises the book on the verge of total irrelavance for anything other than historical research. This is emblematic in the greatest problem to my eye: the lack of a 'buailte' (the little dot) over certain consonants and the lack of a following 'h' to indicate the same thing. For those new to the language, this is widely misleading. No one should see "dub" as the translation of 'black' when the modern spelling, "dubh," actually aids in pronunciation, understanding, transliteration to English, and overall relavance.
The strength of the book is its breadth. If you already understand the manners of Irish naming custom, and certainly you'll need exposure to pronunciation, you will make much use of this book. Further, it draws from a number of the best quality sources.
This is certainly not a baby-naming book. Nor is it relavant for anything regarding modern naming conventions. For these subjects I would recommend the small but very dense and accurate, 'Irish First Names,' by Ronan Coghlan and the many sources which draw strongly from or rewrite/edit the works of Father Patrick Woulfe. I don't wish to speculate, but O Corrain & Maguire seem to fly in the face of Woulfe's earlier research. This book is most valuable to SCA members and sometime scholars.