A well detailed, well sourced account of Irish calendar customs, as the book's title suggests; if you're an Irish Recon, or are looking to add historical elements you your Irish Pagan practice in any regard, then obtaining a copy of this book is paramount to understanding surviving Irish customs- though it's certainly not an easy book to get ahold of these days.
It should be integral to note, however, that while modern Pagans of Irish bent often use it for reconstructionist purposes, it
does not
contain any quote unquote "Pagan Vestiges" of practice. Indeed, any idea of "Pagan Vestiges" was properly abandoned by folklorists and historians not long after the field of study was originally established, after the evidence uncovered; by the 1950's the idea fell out of favor near entirely- now surviving almost entirely within the realm of Pagan and Neopagan fantasy. It does not exist. The idea should be abandoned, and the practices detailed in this book should be looked at with the proper respect for what they are and who they belong to.
In that regard, what the book properly details are the Christian (prevailingly Catholic) holidays and their associated practices- both folk and ecclesiastical- among the Irish peoples; specifically the modern 19th and 20th centuries, though the author occasionally mentions earlier dates and practices. This is still quite useful in several manners for modern reconstructionist purposes if one seeks to incorporate legitimate Irish traditions into one's practice. But they should never be disrespectfully interpreted as "Pagan", let alone "Pagan Vestiges".