Who were the Celts?
The Celts were an Indo-European group, that is, related linguistically to the Greeks, the Germanic peoples, certain Italic groups and peoples of the Indian sub-continent. They arose in central Europe at the beginning of the first millennium B.C. and were an iron using and horse rearing peoples. By the end of the first millennium B.C. their cultural group had spread up and down the Danube and Rhine, taking in Gaul, Ireland and Britain, across central Europe, into northern Italy and northern Spain. Their roaming across Europe led some of the Celtic tribes to sack Rome in 390 B.C. (creating a fear of the northern barbarians that was to haunt Romans for hundreds of years to come), and in 279B.C. another Celtic tribe sacked the Greek sanctuary at Delphi, going on to found a Celtic kingdom in Asia-Minor, Galatia (The people to whom St. Paul was to address some of his epistles). Celtic peoples were apparently fierce warriors, with a taste for head hunting and going into battle naked, though armour of varying types are not uncommon artifacts (e.g. the famous Witham Shield in the British Museum).
The Celts are defined archaeologically by the type-sites of Hallstat (Austria) and La Tene (Switzerland), the former being taken to relate to an earlier phase of cultural development. Hallstat, an ancient salt mining area, was excavated from 1876 onwards by the Viennese Academy of Sciences and provided the first classification of the prehistoric Celts. In 1858, the waters of Lake Neuchatel in Switzerland sunk to a low level, revealing a large prehistoric settlement with a huge number of surviving artifacts The nearby town of La Tene gave its name to the second phase of Celtic cultural development (N.B. These phases overlap through time, and according to geographical area).
Linguistic scholars divide Celtic peoples into the so-called Goidelic and Brythonic branches, where the hard c- sound of the Goidels (e.g. the Irish or Scottish "Cenn", head) becomes a softer p-sound in Brythonic (Welsh or Cornish "Penn", head).
The first Celtic immigrants to the British Isles probably arrived between 2000 and 1200 BC. These are known as the q-Celts and spoke Goidelic. The label q-Celtic stems from the differences between the early Celtic and Italic (i.e. the language that developed into Latin) languages, which included the lack of a 'p' in Celtic and an 'a' rather than the Italic 'o'. A later wave of Brythonic-speaking Celtic immigrants is called the p-Celts.
The three Gaelic languages spoken in Ireland, Man and later Scotland were derived from Goidelic. Welsh and Cornish came from Brythonic. Brythonic survived in mainland Europe in the form of Breton.
The two Celtic language forms share words in common. The difference that gave rise to the labels (pCeltic and qCeltic) is seen in the way the word for horse ekvos in Indo-European, equos in q-Celtic; (cf equus in Latin) epos in p-Celtic.
The term giodelic occurred when the Irish Celts established colonies in present day Wales. The locals inhabitants called the Irish "gwyddel"(savages) and the term came to applied to their language.
Frank Delaney, "The Celts" (Grafton, London 1989: Chapter 1)