There are some interesting things to know. Knowing some of these things helps to explain just how Great Britain finds itself with such a perplexing problem.
An Irish war of independence resulted in an Irish Republic separate from the UK in 1922. The counties making up Northern Ireland were not included in the new republic and remained part of the UK.
Not everyone affected by the Treaty agreed with the partitioning of Ireland.
In 1973, despite some lack of agreement that Ireland should be partitioned, both the UK (including Northern Ireland) and the Republic of Ireland joined the EU, as two previously independent nations.
The Republic of Ireland joined the EU with a remarkable distinction that set it apart from other EU nations. Modifications to the treaties that created the EU could be approved by the governments of the member nations but the Republic of Ireland could only change those treaty agreements by referendum of the voters. This gave Irish voters a direct say in EU governance that other EU citizens lacked.
Now comes the tricky part.
In 1998, Great Britain and the Republic of Ireland signed the Good Friday Agreement to end the troubles in Ireland. One might ask, did the EU agree to the Good Friday Agreement? How could they not involve themselves in an agreement between two of their member states, each of which presumably had the power to leave the EU, that specified that there be no border controls between those two nations?
The answer, of course, is that EU nations do not believe in borders. They are globalists who believe that everybody should just get along and do what the government says to do.
At the very least, the EU should have insisted that Ireland and Great Britain could only join the EU if they agreed that if either one left the EU than both must leave. Allowing just one of them to leave the EU re-creates the very situation that the Good Friday Agreement was meant to solve.
From what I can learn, all the parties to the Brexit mess, including the EU, are getting what they deserve for having treated their national sovereignty with such disrespect.
Since the situation was a mess before Brexit, I don't see any way that it won't be a mess after Brexit.
If more knowledgeable people find that I have misunderstood some part of this I welcome correction.
The EU do have strict borders with non EU countries. That is part of the bone of contention behind the Irish issue now
The EU was very much involved in the GFA talks. Although not a direct participant, it was a present observer and facilitator throughout - and of course the fact that both state parties were EU members helped lubricate the process.
Much of the confusion over the border issue arises from confusion between a 'people' border (passports/immigration) and a goods border (customs). Different rules apply to each.
All EU states belong to a customs union, which is integral to the EU treaties. There was therefore already an open customs border between the Republic and Northern Ireland at the time of the GFA, and customs didn't need to be addressed in the agreement.
The 'people' border is a different matter. There is no EU requirement for passport-free travel between EU states. Only a certain groupf of EU countries (the 'Schengen' group) have mutual passport-free borders, but this is not mandated in the EU treaties. Neither the UK nor the Republic of Ireland are Schengen members. The open people border between Northern Ireland and the south is therefore a bilateral agreement between the two states, not directly relevant to their EU membership.
The problem created by Brexit therefore relates entirely to the customs border. There is no reason why the bilateral open 'people' border should not be retained - indeed the EU has consistently supported the insistence of the Republic of Ireland that it should be retained. But if the UK leaves the EU it ipso facto leaves the customs union and a customs border follows. None of the parties - the UK, the EU, the people either side of the border - want this.
Trying to square this circle has led to the impasse over the 'Irish backstop'.