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.
Gàidhealtachd
For historical reasons, including the influence of a Scots-speaking court in Edinburgh and the plantation of merchant burghs in much of the south and east, the Gàidhealtachd has been reduced massively to the present region of the Outer Hebrides, the Northwest Highlands, the Skye and Loch Alsh and Argyll and Bute, with small Gaelic populations existing in Glasgow and Edinburgh. The Highland Clearances of the 18th and 19th centuries also contributed to the decline of the language, as they reduced the population of the Scottish Highlands, which were predominantly Gaelic-speaking at the time.