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.
Languages of Scotland
Scottish Gaelic and Irish are generally viewed as being languages in their own right rather than dialects of a single tongue but are sometimes mutually intelligible to a limited degree – especially between southern dialects of Scottish Gaelic and northern dialects of Irish (programmes in each form of Gaelic are broadcast on BBC Radio nan Gaidheal and RTÉ Raidió na Gaeltachta), but the relationship of Scots and English is less clear, since there is usually partial mutual intelligibility.