The book offers a comprehensive overview of forms of modern Irish within a general linguistic framework. Starting with information on the sociolinguistics of modern Irish and on the overall sound system of the language, it then proceeds with a tripartite division of the present-day language into northern, western and southern Irish. It gives specific information on the features of each dialect and considers many sub-divisions, using maps and tables to illustrate clearly what is the subject of discussion. There are several innovations in the book, such as a system of lexical sets which facilitate the description and analysis of variation and change in modern Irish.

The data for the book stems from recordings of more than 200 speakers and all the statements made about the structure of Irish are based on native speakers' speech samples. These are supplied online with a software interface which allows users to quickly orient themselves among the varieties of Irish via clickable maps.

A number of further issues are focused on in the book, such as the possibility of dialect reconstruction and the use of place-name evidence for determining the earlier distribution of Irish. Additional historical and background information is provided so that scholars and students without any previous knowledge of the language can readily grasp the themes and issues discussed.