Painting a linguistic history of any place is very difficult where there are no written records. Consequently there is no solid knowledge of the language(s) spoken in the British Isles or Brittany before the Celtic and Roman eras. It is suspected that the language(s) would have not been Indo-European, however.
Only Ireland and the Isle of Man were untouched by the Roman expansion of the first two and a half centuries of the first millennium. All the other Celtic nations saw Latin as a significant language, though in Scotland the impact was relatively short-lived and minimal, as the Romans could not penetrate too deeply into the lands of the tribes that later merged to become the Picts. Regardless, Scotland, Wales, Cornwall and Brittany are all littered with sites of Roman occupation, and inscriptions on their coins, building stones and so on are of course in Latin. And of course it is important to remember that Latin was important throughout the Medieval period, as the language of learning in Western Europe.
But while the Romans were doing their thing, of course the indigenous peoples of what is now the British Isles and Brittany were doing their thing, and there were many other languages being spoken. The Norse made their presence felt in the 9th century, and soon afterwards Norman French was important across the British Isles and in Brittany. English did not become the official language for the English Parliament and courts until 1362.
Ireland
Ireland is relatively straightforward. It is not known what language was spoken there before the Celtic migrations from the continent in the late centuries BC. By the time the Romans arrived in England, the Irish were uniformly speaking a Celtic language. Nothing in particular changed, except that the language evolved into the common ancestral Q-Celtic language. It was not until the coming of the Normans in the 1170s that Irish came into conflict with another language, this time Norman French. Over the years, Norman French was largely confined to pockets around the country and ‘the Pale’, the area radiating from Dublin that changed size according to the extent of control exerted from England. Eventually, English replaced Norman French as the language of administration. Ulster Scots was later brought to the northern parts of Ireland starting in the 17th century, and it was spoken in much of the Ulster area, and still survives in some areas.
