>1. Where can I find in-depth information on the development of the
>Celtic languages (in general) from PIE?
Try Academic Journals such as Etudes Celtiques or Zeitschrift fuer Celtische
Philologie as well as books like:
Russell, P. An Introduction to the Celtic Languages. New York, 1995
Ball, M. J., Fife, James, eds. The Celtic Languages. London and New York,
1993.
as well as:
Lambert, P.-Y. La Langue Gauloise. Paris, Paris, 1995.
>3. How did the vowels and diphthongs develope from Common Celtic
>through Common Brythonic to Old, Middle, and Modern Welsh?
See not only Lambert above, but also:
Jackson, K. H. Language and History in Early Britain. Edinburgh, 1953
>5. Is there any resource material (available to the layperson) that
>discusses (in detail) the phonology of all the Celtic languages (but
>most specifically Irish and Welsh)??
Not much for the layperson - but if you familiarize yourself with the basics
of linguistic terminology, you should be able to get the gist of things in
most of the journals and books mentioned above. I suggest getting a
"dictionary of linguistics" to help you out with the terms and some of the
basic theories.
Once you get that, try this book:
Lewis, H., Pedersen H. A Concise Comparative Celtic Grammar. 2nd Edition.
Goettingen, 1961.
Hope this helps,
C. Gwinn