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.
Gaels
In the 12th century, Normans conquered parts of Ireland (leading to centuries of conflict), while parts of Scotland became Normanized. However, Gaelic culture remained strong throughout Ireland, the Scottish Highlands and Galloway. In the early 17th century, the last Gaelic kingdoms in Ireland fell under English control. James I sought to subdue the Gaels and wipe out their culture; in Ireland by colonizing Gaelic land with English-speaking British settlers, and in the Scottish Highlands via repressive laws such as the Statutes of Iona. In the following centuries most Gaels were gradually anglicized and Gaelic language mostly supplanted by English. However, it continues to be the main language in Ireland's Gaeltacht and Scotland's Outer Hebrides. The modern descendants of the Gaels have spread throughout Britain, the Americas and Australasia.