I'm now experts on Celts, but a brush up was on the Scots-l list a little over a week ago.

Roughly Celts were divided into three language groups, Goidelic, (European and long extinct as a language), Gaelic (Irish and Scots), and Brythonic (Welsh, Manx, Breton).

A Northern Irish group called Scots colonized Argyle and Kintyre and adjacent islands in the 3rd and 4th centuries A.D. establishing the 1st kingdom of Dalriada. A further group of Scots under Fergus mac E-something and two brothers conquered the Dalriada Scots about 500 A.D. and established the 2nd kingdom of Dalriada. These subsequently absorbed and eliminated the Picts, and occupied the highlands and western and northern islands of Scotland (named after these Irish).

John Morris in his history 'The Age of Arthur' notes 3 massive emigrations of Britons (Cymri/ Welsh) to Brittany, the 1st is a little fuzzy, but sometime in the period 380-420 A.D. when Roman rule was disintegrating in England. The 2nd was about 460 A.D. when Vortigen's kindom (under a successor) fell apart, and the 3rd about 520 A.D. when civil war in Wales divided the kingdom. [This may have ended with the battle of Camlann, in which Arthur and Modred supposedly perished).

A further note: 'Welsh' derives from a Saxon word meaning 'foreigner' and isn't from Celtic, and modern Welsh prefer to be called Cymri.