This contemporary map of Europe depicts current boundaries. When early peoples came to the islands area during the beginning of recorded history, the boundaries of countries shown were much different, if there were any boundaries at all. Boundaries seemed to be set up between competing tribes, lines which tended to fluctuate one way or the other or completely disappear, depending upon which tribe was the stronger.
I'll try to locate an ancient map which will depict boundaries six thousand years ago.
In order of their appearance in written history, Early settlers to the islands were Firbolgs of debated European extraction, with some opinions they came from the Middle East or around Greece; Milisians who ostensibly came from Spain; Celts who came from Gaul, now called Normandy; Anglo-Saxons from Normandy and Germany; Romans who came from Rome, now Italy, who ostensibly brought Catholicism to the UK; Vikings first from Denmark then from Norway, Barbarians who came to conquer and plunder; and last the Normans who were purportedly descended from Vikings who settled old Gaul, became Christians and learned to speak French.
So to cut to the chase, they were mostly Vikings and Celts, even the ones from Spain.
Those early peoples seem to have arrived in the UK between 7500 BC and 1200 AD and spread out to settle Ireland, Wales, Scotland, England, and the outer islands.
People who emigrated to the islands since 1100 AD were generally of all faiths and of many races who mainly came for peaceful purposes to trade, or in a few cases, to settle and farm. After the Normans, no other countries besides Spain and Germany ever tried to conquer the Emerald Isles.