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.
Indo-European migrations
Modern Celtic languages are mostly spoken on the northwestern edge of Europe, notably in Ireland, Scotland, Wales, Brittany, Cornwall, and the Isle of Man, and can be found spoken on Cape Breton Island. There are also a substantial number of Welsh speakers in the Patagonia area of Argentina. Some people speak Celtic languages in the other Celtic diaspora areas of the United States, Canada, Australia, and New Zealand. In all these areas, the Celtic languages are now only spoken by minorities though there are continuing efforts at revitalization. Welsh is the only Celtic language not classified as "endangered" by UNESCO.