These Celts spoke two Celtic dialects. No-one is sure whether these two dialects developed before or after these Celts arrived in these islands. The names we call these two dialects are Goidelic and Brythonic. Goidelic was spoken by the Celts of Ireland and Brythonic by the Celts of Britain – apart from the far north where the Picts lived.
To cut a long story short…
The Romans came to Britain in 43 A.D.; they stayed here for 400 years before leaving about 410 A.D. Roman influence over Britain was temporary and mainly over southern parts of the Island. They did not succeed in changing the language or way of life of most of the Celtic people of Britain – the Brythons. They had no influence on Ireland.
From the fifth century onwards Germanic people (Anglo-Saxons) started moving into the south and east of Britain and settled here. These people had a major influence on the life of Britain, especially on the south and east. By the time the Normans arrived in 1066 Britain had changed; in the north were the people of Scotland or Alba, in the west were the Celts of Wales, in the south-west were the Celts of Cornwall (Kernow),and in the east and south were the people of Englaland – England.
From the 13th century onwards the influence of the Crown and Government of England over the rest of Britain and Ireland has been significant; the language, culture and politics of the English has greatly influenced the language, culture and politics of the Celtic people of these islands.