Early Irish History and Mythology was published in 1946. It was a time when not a lot of work was done on Celtic mythology or religion. It is an out dated work that inspired many scholars to follow suit and start the boom in the study of mythology. This is a book that has to be read if you want to follow the progress of theories about the Celts, and also because at the time this book was written nothing could match it in terms of scholastic effort or content.
O’Rahilly is concerned mainly with the early history of Ireland in this book. The book confines itself to the history of Ireland before the official introduction of Christianity in 431 CE. The author’s main source for writing this book is the native Irish tradition, supplemented by the writings of the classical writers and linguistic evidence.
The book is impressive given the time it was written in and the information available to the author at the time. Most of his theories are discarded now but how he arrived at these theories is just as interesting as the theories themselves. This is not a book for beginners though because you’d need to recognize the discarded theories from the ones that are still viable. A word of warning, this book is a bit of a dry read, but it is a valuable one to have nonetheless.