Janista,
Brexit has triggered much nationalism among the 4 member states of the UK. In terms of EU the UK is ONE MEMBER. That UK has 4 parts is immaterial to the EU.
The division arises as although the overall result of 2016 was a majority to leave the EU, both NI and Scotland as components of the UK voted to remain in the EU. However, it is worth noting that in 2014 Scotland voted to remain in UK in greater numbers than voted to remain in EU in 2016. How many of those in Scotland who voted remain in UK were swayed by Cameron's statement that it made EU membership certain can't be measured, but likely have played a part judging by the 2016 vote.
NI is more complicated. For decades there were troubles and violence as many on the island of Ireland did not like the separation into NI and ROI as it was seen to be a continuation of British rule. Go back further to the mid 1800s and potato famine, which some think was an attempted genocide by England, it is easy to understand why relations between Ireland and England are strained.
The GFA was completed in 1998 with the object of reducing hostilities and seems to have been successful in achieving that goal.
That UK, as a single member of the EU, voted overall to leave EU and is going to do so deal or no deal if Johnson is correct is seen by both NI and Scotland as yet another;
"hey you small fry do as you are told by big brother"
So rather than be bossed about forever they are talking of independence from the UK. Nothing wrong with that and some might say it would be a continuation of what has happened over the centuries whereby the British Empire has reduced in size as former colonies have chosen to go it alone. At its peak the empire was 25% of World population and the largest the World has ever seen with a population that is greater than the current EU population.
The impasse arises as honoring both the GFA and the result of the 2016 referendum is not easy to achieve as EU regulations on movement of goods between EU and non EU countries would contradict the GFA in terms of border control between NI and ROI. There is talk of technology solution, but so far nothing has been seen. If someone can invent a technology solution they should be given the Nobel Prize.
I think this is why Johnson has said to EU;
"Remove the backstop or it is no deal"
A no deal means that UK would have to trade on WTO rules. How does that solve the border issue? It does not, but WTO rules do not specify border controls. So Johnson can sit back and say to EU;
"if you want to control goods entering ROI from NI get on with it yourselves"
The underlying strategy being that it would be EU that is tampering with the GFA as opposed to UK.
A suggestion was to make NI a special economic zone. I would have supported that as it seemed to solve everything. However, DUP, ERG and Brexit hardliners did not like that as it was seen to separate NI from UK. Also possible that Scotland would have wanted the same special treatment.
Conclusion is that it is never possible to please everyone at same time. Most likely outcome is that UK leaves EU with no deal and the UK breaks up. NI eventually reconnects with ROI and Scotland votes for independence as UK departure from EU entitles them to another referendum.