Its an interesting idea, but how does it hold up to some scrutiny.
Starting with Scotland. A currently dependent state that is unhappy with an arrangement that sees economic and political decisions made against its explicit consent and/or democratic mandates. Evidence shows its pro EU. Population 5.5m. GDP 205B USD
Wales, also a dependent state with polling indicating perhaps 30% or more of its citizens favor running their own affairs. Possibly now pro EU despite getting caught up in anti EU sentiment. Pop 3.2m. GDP 77B GBP
Ireland. A partitioned 2 part state, with one part dependent on Whitehall per the above, with the other part being a fully independent EU member state. Both parts, evidently, are pro-EU. Pop 5m republic/1.8m NI. 418BUSD Republic /47B GBP NI
All of this is fine enough, at first pass appears like we could be on for a winning idea if the EU was the underlying premise for tying these states together in some sort of federation with a population around 15m. Plenty for a newly constituted federated state. Between them they all have more than enough GDP with Ireland by far and away the largest single GDP contributor, hilariously enough, given its position a mere century ago.
But the next bit is where the idea runs in to real problems.
NI is virulently UK Unionist, with the test of this statement arriving in the coming months with the next election. Scotland too appears balanced on 50:50 Unionist/Independent sentiment. And Wales to my knowledge hasn't yet breached 50% Independent sentiment.
Then we get to Whitehall/Westminster. The Legislature and Administrative centers of the current constitutional settlement are beyond any shadow of a doubt Unionist and of course have a significant say over the make up of the state, as do any such centers.
To reinforce the point, we have witnessed and are witnessing and expression of a new form of political ideology within these centers. One that simultaneously rejects both Devolution and the EU for a vision of a tightly bound UK with international trading relations around the world.
It is not uncommon in the press to read views suggesting Scotland be stripped of all and any means to cease the Union its aristocratic elites signed it up to in 1707, and this in Tom Devines words, is new and unprecedented in the history of the Union (1707).
With the Single Market Act as recently passed, many aspects of Devolution have already been rolled back with more likely to follow.
And so we get to the heart of the matter.
By what means can we even entertain the idea of a Celtic Union in the present political climate? Whitehall/Westminster are overtly saying they will not "allow" a Scottish referendum.
Moreover, hypothetically, should Wales and Scotland cease their respective Unions with England, what evidence do we have to hand to suggest they are going to volunteer sovereignty to such a block/state?
We wouldn't be having this conversation if it were not for Brexit, and there was a time that reversing Brexit may well have mitigated these sort of discussions.
After the last few years and with the most recent government, sentiment against the UK Union appears to be on a one way trajectory. Regardless of what mechanisms or processes are implemented, the various, mostly Celtic, nations of the UK state cannot be full democracies or govern themselves or their economies as they may wish. Even parts of England are going down this path now too.
Whats telling is that Whitehall/Westminster appear to be doubling down. Unwilling to admit the imbalances that exist, created by Whitehall/Westminster no less, they are seeking to hold the state together by force against the trend of diminishing support. Even within England support for the Union has been shown by polling to be limited. This is telling because the very thing that Scotland/Wales/Ireland dislike/ed about the Whitehall/Westminster system is its overarching need for power and total authority, regardless of what these countries wanted or needed.
It really does appear that a Leopard cannot change its spots.