The racial mixture of the populations of the British Isles is highly complex; largely due to the continued movement of significant portions of the population throughout recorded history. The population of Scotland in the 16th and early 17th centuries was made up from the remnants of the early Celtic inhabitants of the British Isles, of Roman invaders and settlers, the Angles, Jutes, Saxon and Viking invaders of the Dark Ages from continental Europe, later Flemings from the Low Countries and the Normans (themselves of Viking origin) who came north after the conquest of England in 1066. The Irish population at the same time was a mix of the early Celts, Picts and other Hibernian invaders plus Viking and other incomers.
The movement of people between Scotland, England and Ireland over the centuries has been driven by a variety of pressures; political, economic, family ties, religious issues and the problems suffered during times of all types of armed conflict and war. There is also a very long history of such movements from the days of the Viking invaders in the Dark Ages of the 5th to 7th centuries up to the times of the potato blight in Ireland of the mid 19th century.
Some movements have sometimes been loosely referred to as clearances; there were several actual clearance campaigns in Scotland and in Ireland, conducted either directly by the English/British Crown or by substantial land-owners on with the tacit support of a benign government. Although history and romantic fiction tend to focus on the Highland Clearances in Scotland, they were effected across the whole of Scotland; evicting, if not wiping out, the resident Highlander, Lowlander or Borderer populations. Similarly, the massive movement of people from Ireland before, during and after the potato blight and consequent famine (the Great Hunger) of the 1840s has attracted much interest; sometimes obscuring substantial but otherwise routine movements at other times.
The "dark romance" of such Celtic evictions obscures the scale of movements of people from other parts of Britain and Europe. There are clearly documented forced migrations of people, especially religious minorities or economically disadvantaged classes, from the south west of England to the Americas and from Kent to Australia. A remarkably similar modern campaign, the forced emigration of orphans from all parts of Great Britain to Australia, only ceased in the 1950s.
It should be recalled that the West Coast of Scotland has a mass of sea- lochs and two belts of islands; the Inner and Outer Hebrides. Furthermore, the County Antrim and County Down coasts of Ireland are very close to Scotland, north-west England and the Isle of Man. The local fishing industry, small-scale trading and the historic movements of populations ensured ready movement by sea between what are now seen as separate countries. In times of crisis, famine or war it was sometimes safer to move family and flocks to another safer or more economically attractive residence. Such escapes were often followed within a generation by a return to the original homeland once conditions there had returned to normal. It is understandable that the family histories of many of the surnames represented in Scotland, Ulster and even Northern England are quite confused.
In an extreme example of routine movements, it should be recalled that in the 18th and 19th centuries, the people of St Kilda (some 40 miles into the Atlantic, west of the Outer Hebrides) were prepared to row to Uist or Harris, against prevailing winds and seas, in order to trade their sole produce, the down of the island's sea-birds for use as mattress filling.