If you’re looking for a good, relatively recent (1995), concise but not too concise, introduction to Ireland from the introduction to Christianity to the 12th century invasion, you could do far worse than this book.
Just under 300 pages plus another 50 or so of resources (glossary, suggestions for further reading, bibliography), it packs a lot into a relatively small package. For comparison, I have two others on the shelf, waiting to be read, at roughly 1000 pages each.
There were places I wanted more detail. For instance on page 39, he describes briefly that “the pre-Christian practice […] reckoned time in terms of three-, five-, ten-, or fifteen-day periods, based on the lunar calendar […] the seven-day week was entirely unknown.” This is fascinating, a huge entrypoint into how people thought about their world and their place within it, but the author gives us nothing further. Bummer.
But overall, it’s a quick, nice introduction to the subject, accessible, even droll in places: “Modern scholars do not quite know what to make of Virgil. Is he just a dotty professor, like many of themselves, and therefore to be humoured?” (212). Very few pages went by without me underlining and/or annotating something in the margin, which is usually a sign of book in which a good time was had by all.