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.
Insular Celts
The Goidelic branch would develop into Primitive Irish, Old Irish and Middle Irish, and only with the historical (medieval) expansion of the Gaels would it split into the modern Gaelic languages (Modern Irish, Scottish Gaelic, Manx). 
Common Brythonic, on the other hand, split into two branches, British and Pritenic as a consequence of the Roman invasion of Britain in the 1st century. 
By the 8th century, Pritenic had developed into Pictish (which would be extinct during the 9th century or so), and British had split into Old Welsh and Old Cornish.