This is based off a standardization of Irish. The three main dialects are Ulster, Connacht, and Munster. The writing and audio dialogues are from speakers from County Galway and utilize the Connacht accent. This was told to me from an Irish-born priest in 2010.
A major example of this is:
1. How are you:
The book uses: Conas tá tú? (this is a standardization mutually intelligible in all Gaeltacht areas)
Here are the variations of 'how are you?':
Munster (southern Ireland): Conas tánn tú?
Connacht (mid-western Ireland): Cén chaoi a bhfuil tú?
Ulster (northwestern Ireland): Cad é mar atá tú?
The book I think is good but can be overwhelming at times. The book sometimes moves too fast and makes self-study challenging. Overall, I would recommend this to anyone interested in speaking, reading, and listening to Gaelige. Thanks!