History of Scottish Gaelic
All surviving dialects are Highland and/or Hebridean dialects. Dialects of Lowland Gaelic have become defunct since the demise of Galwegian Gaelic, originally spoken in Galloway, which seems to have been the last Lowland dialect and which survived into the Modern Period. By the 18th century Lowland Gaelic had been largely replaced by Lowland Scots across much of Lowland Scotland. According to a reference in "The Carrick Covenanters " by James Crichton, the last place in the Lowlands where Scottish Gaelic was still spoken was the village of Barr in Carrick: only a few miles inland to the east of Girvan, but at one time very isolated. Crichton gives neither date nor details.
Celts (modern)
It is these six nations that (alone) are considered Celtic by the Celtic League and the Celtic Congress amongst others. These organisations ascribe to a definition of Celticity based mainly upon language. In the aforementioned six regions, Celtic languages have survived and continue to be used to varying degrees in Ireland, Scotland, Wales, Isle of Man, Cornwall and Brittany. There are also Celtic nomads: Irish Travellers called "Pavee" that speak a language called Shelta that is a creole of Irish Gaelic and other languages and Indigenous Highland Scottish Travellers called "Tinkers" who speak a language called Beurla Reagaird that is an acrolect of Scottish Gaelic.