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.
Pan-Celticism
Before the Roman Empire and the rise of Christianity, people lived in Iron Age Britain and Ireland, speaking languages from which the modern Gaelic languages (including Irish, Scottish Gaelic and Manx) and Brythonic languages (including Welsh, Breton and Cornish) descend. These people, along with others in Continental Europe who once spoke now extinct languages from the same Indo-European branch (such as the Gauls, Celtiberians and Galatians), have been retroactively referred to in a collective sense as the Celts, particularly in a wide spread manner since the turn of the 18th century. Variations of the term "Celt", such as "Keltoi" had been used in antiquity by the Greeks and the Romans to refer to some groups of these people, such as Herodotus' use of it in regards to the Gauls.