Six nations tend to be most associated with a modern Celtic identity, and are considered 'the Celtic nations'. These are:
Ireland, Scotland, Wales, Brittany, Cornwall, the Isle of Man
It is these 'Six Nations' that (alone) are considered Celtic by the Celtic League, and the Celtic Congress. These organizations ascribe to a definition of Celticity based upon language. In the aforementioned six regions, Celtic languages have long survived and in the cases of Ireland, Scotland, Wales, and Brittany continue to be used.
A number of activists on behalf of other regions/nations have also sought recognition as modern Celts, reflecting the wide diffusion of ancient Celts across Europe. Of these, the following regions are prominent:
Galicia, Asturias, England
In neither Galicia nor Asturias has a Celtic language survived, and as such both fall outside of the litmus test used by the Celtic League, and the Celtic Congress. Nevertheless, many organizations organized around Celticity consider that both Galicia and Asturias "can claim a Celtic cultural or historic heritage." . Justification for these claims to Celticity derive from similarities in music, dance, folklore, and culture to the other widely recognized Celtic countries. Further support is drawn from findings that the populations of northwestern Spain and those of other Celtic regions share a common genetic makeup.The Longue DurÃ©e of Genetic Ancestry: Multiple Genetic Marker Systems and Celtic Origins on the Atlantic Facade of Europe, American Journal of Human Genetics
England has a wealth of Celtic music, dance, folklore and culture, and the Cumbric language was identified with northwestern England until long after the Norman invasion. England as a whole comprises many distinct regions, with some acknowledging a larger Celtic identity than others.
Comment: It may come as a surprise to many readers of this blog but England can be considered to be a Celtic nation as the population is largely descended from Celtic forebears. According to the latest research there is more genetic evidence from Vikings in the north-east than from Angles and Saxons in the South. Most of the Germanic peoples settled in the area ruled by Offa in the Central Midlands and East Anglia. 67% of the DNA of England as a whole is considered Celtic with larger proportions in the north-west and south west and Kent and smaller proportions in the centre (Mercia) and the east.