The Celtic tribal groups that settled among the islands (Hibernia, Albion, and Isle of Man) scattered about and formed their own distinct languages. Hibernia (Ireland) was the seat of the Q-Celtic tongue, and Albion (Britain) was the seat of the P-Celtic tongue. The figure to the right shows a diagram of the rough chronology the roots of these tongues. The "Gaelic" tongues include Irish, Scottish, and Manx. The "Brythonic" tongues include Welsh, Cornish, and Breton. They all share a common root back to the Celts that occupied central Europe. What a deal!
The following reference have been most helpful in the study of this chronology:
1) The Celtic World, Edited by, Miranda J. Green, first published in paper back 1996, Routledge, London.
2) The Celtic Empire, The First Millinnium of Celtic History 1000 BC - 51 AD, by Peter Ellis, Carolina Academic Press, Durham, NC, 1990.
3) The Celtic Realms, The History and The Culture of The Celtic Peoples From Pre-History To The Norman Invasion, by Myles Dillion and Nora Chadwick, Castle Book, Edison, NJ, 2006.
4) The Ancient World of the Celts, by Peter Ellis, Barnes & Noble Books, NY, 1998.
5) Historical Atlas of the Celtic World, by Angus Konstam, Mercury Books, London, 2001.
6) The Celts, Uncovering the Mythic and Historic Origins of Western Culture, by Jean Markale, Inner Traditions International, Rochester, VT, 1978. [first published in French, Les Celts et al Civilisation Celtique, 1976.]
7) Celtic Britain, by Lloyd Laing, Charles Scribner's Sons, NY, 1979.
8) Celtic Ornament in the British Isles, by E.T. Leeds, Dover Publications, Inc., NY, 2002.
9) A Picture Book of Ancient British Art, by Stuart Piggott, and Glyn Daniel, Cambridge, At The University Press, 1951.
10) The Life and Death of A Druid Prince, by Anne Ross and Don Robins, Touchstone, NY, 1989.
11) The Story of The Isle of Man, Vol. 1, The Earliest Time to 1406, by C.W. Airne, Liverpool, 1949.