Your challenge is to come up with an AH scenario in which all of the 'Celtic Nations' (Ireland, Scotland, Brittany, Wales, Cornwall and the Isle of Man) are united under a single pan-Celtic government by the present day. This Celtic Nation must also exclude England (or the majority thereof); and you get bonus points if you can pull this off with a POD later than 1838, after the first meeting of the Celtic Congress had taken place in Abergavenny. Up for it?
Can it include England if it was a majority Celtic England?
Hrmm, Rivals of Great Britain (France, Russia, Austria, Whoeveryouwannachoose) support a group of Celtic Supremacists, or manufactuer some. By building a support base of disenfranchised Irish who are angry about British rule, blaming the British for all their issues, ferment rebellion supported by their powerful backers, who just want to see England suffering.
Russia could be a good candidate to support this as a distracting for GB during the Great Game in Afghanistan. But better for my idea could be Austria. A Catholic Celtic nation could be seen as useful to the Austrians.
The Irish movement gains momentum, and starts fostering Celtic Supremacy groups in Wales, Britanny and Scotland. Imagine this as a consistent movement over 50+ years.
Using Russian money, and taking advantage of British free trade laws, they acquire as many arms as they can, waiting for Britain to suffer.
Assuming a butterfly net, they finally launch their rebellion during the First World War once Britain and France are entrenched against Germany. Joining the war on Germany and Austrias side, they cause major issues on the British Isles, and with the British in Europe, they cause Germany and Austria to succeed in the war, who in turn recognise the Celtic states independence as Britanny, Eire, Scotland, Cornwall and Wales, who unite under a Celtic Union, who begin to systematically deport Anglo-Saxons from their territory, and later parts of occupied England (think Wessex/Upto the Severn/Down to the Tyne).
I can't say it is the most thought through, but a Pan-Celtic supremacy movement seems the best bet for a modern-ish PoD.
Otherwise, Stop Julius Caesar invading, have a Celtic warlord unite Britannia and Hibernia and build a strong navy, but ally with Rome in the long term.