Cons: the book is poorly structured. There might be more tables and the rules might be spelled out more consistently; Pros: the keys to both texts and exercises (and this is my very first book on the subject :))
If you want to learn to speak Irish, this is the book and tapes for you. To me without the tapes it would take twice as long. The language is a difficult one. For many reasons. And as another reviewer stated there are so many letters in words that are silent. The pronunciations are very different from ours, for example the authors name is pronounced Meal O'Sheal. But this book is so detailed they even tell you how to place your tongue in your mouth when pronouncing sounds. I started to take notes on index cards to help me, but everything I tried to short cut that way was already listed exactly that way somewhere in the book already as a quick reference guide. It's not something you'll learn over night, but this will definately teach you.
If you want to learn actual Irish as it is spoken and not the standard (which isn't spoken -anywhere- except by non-natives), this is one of the two best books. This text teaches Irish as spoken in Cois Fharraige, in the Connemara Gaeltacht. It is very thorough and can come off as dry and academic, but is still one of the best resources for actual Irish if the learner is willing to put the time in.
For Munster Irish, the 1961 version of Teach Yourself Irish cannot be beaten. It teaches West Cork Irish from native speakers, and can be found online (it's out of copyright) with audio.
If you wish to learn Ulster Irish, I don't know many beginner resources for you, but I have heard that Buntús na Gaeilge, offered free on Dr. Barbara Hillers' website, is a good start.