Stephen Lawhead put a pronunciation guide to names in his Song of Albion Series. The series is about a graduate student from Oxford who slips though a portal and winds up in ancient Brittan. It's basically a retelling of the story of King Nuadu, the Silver handed King of the Tuatha De' Danann. His work pre-dates yours by several hundred years, but it still deals with early Celts. If Lawhead, who is a very successful author, and his publisher, found it acceptable then I see no reason you should abandon the idea. Most people don't understand the differences in pronunciation between the Celtic languages and English.
Off topic, but maps can be useful even without "borders." After all, we know where the Boii lived from as early as the bronze age, the same for the location of the Nori, the Amber road, principal mines at Hallstatt, regions of smelting for iron and bronze, etc. It may be handy for your readers to know which clan lived where. If you are interested, I'd recommend, Atlas of the Celtic World, by John Haywood. It's published by Thames & Hudson, Ltd. of London. The back 1/3 of the book is dedicated to Celtic Brittan and has some fantastic maps. (ISBN: 0-500-05109-7)
