First, a couple of admissions:
1. I'm not an academic; I have no professional qualifications to judge this book. But I enjoy learning languages, and I know what works for me.
2. I'm only up to lesson 9.
Several years ago, I took a stab at learning Old Irish, using a combination of Lehmann, Quin, and Strachan. I didn't get too far. It's an extremely difficult language, and it has an exotic, almost non-IndoEuropean look to it. But with this book, the dedicated learner should be able to reach what looks like a fairly sophisticated reading proficiency in Old Irish.
After a couple of introductory chapters, the author gets down to business with Chapter 3 (phonology and orthography) and Chapter 4 (overview of the initial mutations). Don't rush pass these chapters in your haste to get into the meat of the grammar; unless you have a sound basis in the phonology, spelling, and sound changes, you'll have trouble down the road.
There are a total of 58 lessons - each lesson introduces several grammatical points. Most lessons have extensive exercises, and translation work from English to Old Irish, and vice-versa. Best of all, the answers to the exercises and translation sentences are provided in the rear of the book.
If you enjoy the intellectual challenge of learning an ancient language, you'll love this book. And it will probably give you that enjoyment for the better part of a year - there's that much material packed into this book. Just a great, great book. It won't make the learning of Old Irish easy (nothing can do that). But it makes it accessible and do-able, if you've got the patience and perseverence.
A nice touch - there are dozens of whimsical little drawings of sheep scattered throughout the book. I don't know what prompted the author to put them in, put I'm glad he did. And you will be, too.