Malachy McCourt is always fun to read, very conversational like a chat on a barstool. I never gave much thought to Danny Boy and where it came from so I certainly never knew it was written by an English lawyer. Throws in interesting look at Derry v Londonderry and being Catholic in Northern Ireland.
The book is interesting, but most of the information within is more of an overview of the song's influence rather than any real insights. It reads like an extended essay. You can probably find just as much information with a quick web-search.