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
What is known as Scottish Gaelic is essentially the Gaelic spoken in the Outer Hebrides and on Skye. Generally speaking, the Gaelic spoken across the Western Isles is similar enough to be classed as one major dialect group, although there is still regional variation. For example, the slender 'r' is pronounced in Lewis, where the Gaelic is thought to have been influenced by Norse, and had a pitch accent system.