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.
Celtic Revival
The Welsh language has been spoken continuously in Wales throughout recorded history, and in recent centuries has been easily the most widely-spoken Celtic language. But by 1911 it had become a minority language, spoken by 43.5% of the population. While this decline continued over the following decades, the language did not die out. By the start of the 21st century, numbers began to increase once more.