The term Celt refers to a number of people from Europe, the ones that used Celtic languages, which could be derived from Indo-European languages. While today the term Celtic is used to describe peoples from Ireland, Scotland, Wales, Cornwall, the Isle of Man, and Brittany, the Celtic languages, Irish, Scottish Gaelic, Manx, Welsh, Breton, and Cornish, is a more accurate association.
The Celtic language first appears in history abound 600 BC, but by then the language had already split in to several groups which are now spread out among Central Europe, the Iberian peninsula, Ireland and Briton. Some claim that the Celtic peoples are of the same genetic ancestry as the Basque people, while others believe that the Celts are a distinct branch of the Indo-European family. The Indo-European culture was dominant in the Bronze Age, from about 1200 BC to 700 BC. This fits into the timeline that puts the Celtic language around 600 BC.
This link will provide some details about early Celtic existence.
http://www.britainexpress.com/History/Celtic_Britain.htm
