British Isles
Anglo-Saxons arrived as Roman power waned in the 5th century AD. Initially, their arrival seems to have been at the invitation of the Britons as mercenaries to repulse incursions by the Hiberni and Picts. In time, Anglo-Saxon demands on the British became so great that they came to culturally dominate the bulk of southern Great Britain, though recent genetic evidence suggests Britons still formed the bulk of the population. This dominance creating what is now England and leaving culturally British enclaves only in the north of what is now England, in Cornwall and what is now known as Wales. Ireland had been unaffected by the Romans except, significantly, having been Christianised, traditionally by the Romano-Briton, Saint Patrick. As Europe, including Britain, descended into turmoil following the collapse of Roman civilisation, an era known as the Dark Ages, Ireland entered a golden age and responded with missions (first to Great Britain and then to the continent), the founding of monasteries and universities. These were later joined by Anglo-Saxon missions of a similar nature.

Q: What era did Ireland enter when the Roman Empire ended?
golden age