The movement of people between Ireland and Scotland has been going on forever and has never stabilised (see earlier comments on other threads). We are finding out more all the time from archaeology and DNA studies, but we shouldn't ignore early accounts either. Sure - they can be wrapped up in myths and heroic legends, but there is truth there also.
I have always been intrigued by the comments by Tacitus in the Agricola.
"Who the first inhabitants of Britain were, whether natives or immigrants, is open to question...But their physical characteristics vary and the variation is suggestive. The reddish hair and large limbs of the Caledonians proclaim a German origin; the swarthy faces of the Silures (South Wales), the tendency of their hair to curl and the fact that Spain lies opposite, all lead one to believe that Spaniards crossed in ancient times and occupied that part of the country. The people nearest to the Gauls likewise resemble them."
Interesting perspective revealed there where Wales is regarded as being opposite Spain. This is supported by recent DNA studies which show links between the Irish, Spanish and Portuguese. The Romans didn't get to Ireland, but from their perspective, Ireland was also opposite Spain. How many modern Scots (Caledonians and therefore the Picts) would consider that they were of German origin and therefore cousins to the equally Germanic Saxons? How many Belgiums and Frenchmen are aware that the Belgae and the Franks were also Germanic? Way off topic now, but the migrations of people is endlessly fascinating
