The Celtic languages are a branch of the Indo-European languages. They were once spoken throughout Western Europe, but are now confined to the British Isles and Brittany. There are two branches: Goidelic or Gaelic and Brythonic or Britannic. The former are represented by the modern languages of Irish Gaelic, Scottish Gaelic and Manx. The second group includes Welsh, Cornish and Breton.
The prospects of survival for the remaining Celtic languages are not good, as decline for all in favor of English (or French in the case of Breton), which had already begun in the Middle Ages, has been tremendous, for example due to economic reasons; as for the speakers of Celtic languages, English and French simply offered them a whole lot more of opportunities.
The last native speaker of Manx died in 1974, while Cornish came close to extinction at the end of the 18th century. However, since de 20th century, both Cornish and Manx have been revived with some success. 
- Daniel Nettle & Suzanne Romaine, Vanishing Voices:The extinction of the world's languages, 2000