Galwegian Gaelic
George Buchanan, himself a Gaelic speaker, writing in 1575, reported that Gaelic was still spoken in Galloway. In the middle of the century, 1563–1566, an anonymous English military investigator reported that the people of Carrick "for the most part specke erishe".
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.