In the battle of Nedao, 454 AD, Attila the Hun's son Ellac escapes with 2000 loyal Huns (instead of being killed). He leads his small army across Europe to Brittany, adopts Christianity and defeats the King of the Britons Riothamus, founding the Kingdom of Brittany. Several years later, Ellac conquers the Irish kingdoms of Mumha (Munster) and Laigin (Leinster). By 500 AD he has unified Ireland under Hunnic rule (in addition to Brittany).
By 700 AD most of the British Islands are ruled by Ellac's dynasty. After the Vikings invade Lindisfarne in 793, a massive punitive expedition is sent by the Kingdom of Brittany to Scandinavia. This dissuades the Vikings from ever invading Europe again. Instead, they turn their ambitions west. America is discovered in 848 AD, and the Vikings make numerous settlements there throughout the ninth century.
Meanwhile, no Viking intervention in Europe means the Kingdom of England and the Kievan Rus' never come into existence. Instead, the Celtic Kingdom of Brittany (the Huns have been assimilated by now) dominates Britain, and Rus' is split between various states, including the Bulgarian Empire and the Khazar Khaganate.