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.
History of Scottish Gaelic
There is no evidence from place names of significant linguistic differences between, for example, Argyll and Galloway. Dialects on both sides of the Straits of Moyle (the North Channel) linking Scottish Gaelic with Irish are now extinct, though native speakers were still to be found on the Mull of Kintyre, Rathlin and in North East Ireland as late as the mid-20th century. Records of their speech show that Irish and Scottish Gaelic existed in a dialect chain with no clear language boundary.