Source 2: http://www.druidsgrove.com
The traditional Celtic nations, where Celtic civilization achieved its height, and where an indigenous Celtic language was spoken, are Alba (Scotland), Breizh (Brittany, or Gaul, what is now France), Cymru (Wales), Eire (Ireland), Galatia (northern Spain), Kernow (Cornwall), Mannin (Isle of Man), and Britain. The Celtic culture was a tribal society, meaning the basic social and political unit was the extended family. They had Iron-age technology at the height of their achievement, meaning they could forge iron for their tools, use gold and silver for art, clear land for agriculture and animal husbandry, and lived in settled farm stead communities. The Celtic people migrated from the ancient indo-european homelands in eastern Europe, to span most of western Europe. It is possible to trace the migration routes by examining the artifacts they left behind. Two classes of Celtic artifacts, La Tene and Halstadd, are named for towns in which artifacts from each period were discovered: Halstadd is in the Salzkammergut in Austria, and La Tene is in Switzerland. The Celts of Galatia, in what is now Turkey, was visited by Paul of Tarsus around 40AD; his epistle to them has a permanent place in the Christian Bible.
The Celts of Scotland were Irish colonists, Scots, and also indigenous, possibly pre-Celtic people known as Picts, who had a matrilineal kingship pattern, and who dominated Scotland until united with the Scots of Dalriada by Kenneth Mac Alpine in AD 843.
Unfortunately, of the Picts, little is known; even their name is the word the Romans used for them and not the name they used for themselves. ("Picti", meaning "painted people", was their epithet from the Romans, because Pictish warriors used to paint themselves blue with an extract from the woad plant when in battle. Some Pictish artifacts, mainly carved stones, do remain, although their symbols cannot be fully understood.
In modern times, strong Celtic cultural centers can be found in Newfoundland and Nova Scotia, Canada, some parts of New England, USA, and Australia.
One Gaulish Celtic tribe worth an honorable mention is the Helvetians, who fought against Julius Caesar's armies in 58BC. Their territory is in what is now Switzerland, and they live on in that modern nation: the official name of Switzerland is still 'Confoederatio Helvetica' (latin for 'the Helvetian Confederation').
map from http://www.sheilascelticwisdom.com