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.
British Iron Age
The tribes living in Britain during this time are often popularly considered to be part of a broadly Celtic culture, but in recent years this has been disputed. At a minimum, "Celtic" is a linguistic term without an implication of a lasting cultural unity connecting Gaul with the British Isles throughout the Iron Age. The Brythonic languages spoken in Britain at this time, as well as others including the Goidelic and Gaulish languages of neighbouring Ireland and Gaul respectively, certainly belong to the group known as Celtic languages. However it cannot be assumed that particular cultural features found in one Celtic-speaking culture can be extrapolated to the others.