During the Dark Ages, significant Irish settlement of western Britain took place. The 'traditional' view is that Gaelic language and culture was brought to Scotland, probably in the 4th century, by settlers from Ireland, who founded the Gaelic kingdom of Dál Riata on Scotland's west coast. This is based mostly on medieval writings from the 9th and 10th centuries. However, recently some archeologists have argued against this view, saying that there is no archeological or placename evidence for a migration or a takeover by a small group of elites. Due to the growth of Dál Riata, in both size and influence, Scotland became almost wholly Gaelic-speaking until Northumbrian English began to replace Gaelic in the Lowlands. Scottish Gaelic remained the dominant languages of the Highlands into the 19th century, but has since declined.
Before and during the Gregorian mission of 596 AD, Irish Christians such as Columba (521–97), Buriana, Diuma, Ceollach, Saint Machar, Saint Cathan, Saint Blane, Jaruman, Wyllow, Kessog, St Govan, Donnán of Eigg, Foillan and Saint Fursey began the conversion of the English and Pictish peoples. Modwenna and others were significant in the following century.
Some English monarchs, such as Oswiu of Northumbria (c. 612 – 15 February 670), Aldfrith (died 704 or 705) and Harold Godwinson (died 1066) were either raised in or sought refuge in Ireland, as did Welsh rulers such as Gruffudd ap Cynan. Alfred the Great may have spent some of his childhood in Ireland.
In the year 902 Vikings who had been forced out of Ireland were given permission by the English to settle in Wirral, in the north west of England. An Irish historical record known as "The Three Fragments" refers to a distinct group of settlers living among these Vikings as "Irishmen". Further evidence of this Irish migration to Wirral comes from the name of the village of Irby in Wirral, which means "settlement of the Irish", and St Bridget's church, which is known to have been founded by "Vikings from Ireland".
Irish people who made Britain their home in the later medieval era included Aoife MacMurrough, Princess of Leinster (1145–88), the poet Muireadhach Albanach (fl. 1213), the lawyer William of Drogheada (died 1245), Máel Muire Ó Lachtáin (died 1249), Malachias Hibernicus (fl. 1279–1300), Gilbert Ó Tigernaig (died 1323), Diarmait MacCairbre (executed 1490) and Germyn Lynch (fl. 1441–1483), all of whom made successful lives in the various kingdoms of Britain.