"The Celtic Heroic Age" is billed as an introductory text for Celtic studies, but I would be nervous about that without more of an understanding of the world in which it's set. It's a series of translations of Celtic texts and references to them in Classical lore. In almost all cases, it's just English, though some of the shrine inscriptions have the original tongues side by side. The translations are very prosaic, as far as I can tell. They convey the information just fine, but for poetic resonance, Kinsella eats them for breakfast. The focus here seems to be on rendering the text as literally as possible, not in telling a good story. That's fine by me -- the book is intended as a reference, and it's often extremely useful to have a minimally interpreted version.
Reading the Classical sources, it helps *immensely* to have an understanding of the biases and agendas of the Classical world, and to have read some of the archaeological references. It helps you separate historical fact from Roman propaganda. The text does not do this for you; it's your job to figure out that Caesar was trying to conquer Gaul, and that these missives were his newsletters to home. There's no background on what the Greeks thought of barbaroi in general, or what qualities they esteemed before they condemned people in entirely unflattering terms. Knowing these things will help set the texts in perspective.
I enjoyed rereading new (to me) translations of many familiar stories. I still have a hard time wading through all the repetitive descriptions of each warrior's clothes, chariot, leg hair, etc., but it's at least less dull now that I catch some of the allusions and intended associations/meanings each time through. I would recommend this book as a reference to Celtic geeks -- it is damn handy to have so many source texts in one place. It's predominantly Irish texts, so it's most useful to insular Celtic folks, but there are Welsh texts and a scattering of Continental inscriptions and references in there too.