TDLR: After peace and stability in the West goes to hell, the UN collapses, and nationalism arises. Ireland and Scotland return to being fully Celtic again while England becomes a 3rd world country wracked by civil wars that the Irish, Scottish, Welsh and Cornish invade to make into a Cornish and Welsh-speaking nation. This event, in turn, prompts a wave of pan-Celtism across Europe in countries where Celts once lived and the creation of Celtic macronations speaking Celtic conlangs arise.
Why does this happen?
In the distant future, worsening racial relations and mass immigration have sparked significant instability, the UN's collapse and the rise of right-wing nationalist governments in the West. The few "winners" of this contest were Ireland and Scotland, which kept a good chunk of their infrastructures intact and enacted a successful and aggressive "re-celticisation" that saw Irish and Scottish Gaelic becoming dominant languages again.
Yet, the Irish and Scottish governments didn't rest on their laurels as they had developed a "never again" mindset deeming their nearest neighbour, England, as "an existential threat to Celtism" due to its imperialist history. With England degraded into a 3rd world country after fighting its constant civil wars, Ireland and Scotland launched a joint invasion of England with help from Welsh and Cornish guerrillas. The invasion succeeded, and England became the Lloegyr Commonwealth, a Welsh and Cornish-speaking nation under the Irish and Scottish rationale of:
"We almost had our languages and cultures destroyed under English imperialism. Now, you'll know what it's like to have your language and culture destroyed under Celtic imperialism!"
But, while the international reaction to the Scot-Irish-Welsh-Cornish victory over England and Lloegyr Commonwealth's creation, consisted mixture of panic and amazement, no-one knew what would come next. In a wave of pan-Celticism that would become known as the "Second Celtic Revival", where many Celtic micronations named after various Celtic clans and speaking languages reconstructed from historical Celtic languages would pop up across Europe in countries with a historical pre-Roman Celtic presence. At first, such a trend was viewed with mild amusement and then annoyance as more of these micronations formed and became macronations, which led to the formation of the United Celtic Coalition, an alliance headed by Ireland, Lloegyr, Scotland, Brittany, and the Isle of Man.
But the question is: why and how would something like this happen? Especially in nations that haven't even been Celtic for millennium?