It was a subject never mentioned during the pre-referendum debate on leaving the EU but subsequently has become a controversial issue causing hostility between London, Belfast, Dublin and Brussels, threatening to derail the process and cause the UK to exit without a favourable deal.
It is, of course, the Irish border - the only land frontier between the UK and the EU - and as Prime Minister Theresa May tries to rescue her withdrawal agreement as the March 29 Brexit deadline approaches, a timely publication is hitting the bookshops.
The Border: The Legacy Of A Century Of Anglo-Irish Politics is written by Diarmaid Ferriter, Professor of Irish Modern History at UCD (University College Dublin). He is also a regular television and radio broadcaster and newspaper columnist.
Q. Why did you decide to write this book?
A. I have always been fascinated by the border. It is a fascinating subject but we are also approaching the centenary of the state in Northern Ireland and Brexit came along.
I wanted to write something that would be accessible. There is a fair degree of ignorance about the border and part of my mission is to enlighten without overwhelming.
Obviously Brexit has put the border centre stage. Ironically there was no discussion about it during the referendum on whether to leave the EU or not and that tells us an awful lot about what people in Britain thought or knew about the border.
Q. Do you think it is a good thing that the border is now attracting so much attention?
A. It is a good thing in that it has provided a reminder that neither Britain nor Ireland can ignore a century of history. There is a tendency on the part of some in the Tory party to classify the issue as the tail wagging the dog, but what it is doing is proving how difficult an issue it is to solve.
But it is also a bad thing in that it has brought back to the light a lot of issues we hoped had been solved or that people had reached an accommodation on.
Q. What has worried you?
A. The coarsening of the rhetoric and the provocations from all sides have reawakened many of the old animosities. For example, a couple of years ago Peter Robinson (former DUP leader) would have said that north-south relationships in Ireland had never been as strong and it was the same with relationships between the UK and Ireland.
But there is always a undercurrent which could lead to violence and I feel it is important for a British audience to understand the meandering complexities of the border.
Q. Was the border ever meant to be permanent?
A. One of the reasons that Britain got involved in partition was because it wanted to get the Irish/Ulster question out of British politics. Philip Kerr, an adviser to David Lloyd George, wrote: 'It (the Government of Ireland Act) would at least accomplish two essential things: it would take Ulster out of the Irish question and it would take Ireland out of English party controversies'.
The original idea from an English perspective was that the border would be temporary and Ireland would work out its own solution.
Instead, the border hardened very quickly. Irish politicians need reminded what contributed to that situation - lazy rhetoric and a failure to engage with the unionist mentality.
Q. What do Irish politicians and the Republic's population in general think of Northern Ireland?
A. Many people still think of Northern Ireland as a foreign country even though Belfast is just 100 miles up the road from my office here in Dublin. This lack of engagement goes back decades and there is still a very strong element of that today. The DUP does not make the situation any easier by downplaying the complexities of the current situation. As well, the calls for a border poll and putting it centre stage at this time could be seen as very unhelpful. Those making those calls seem to assume that all nationalists in Northern Ireland are screaming for an united Ireland.
Q. Has it always been daggers drawn on both sides of the border?
A. In the 1950s a few Irish writers were posing the idea of consent when resolving differences. They said that there was always going to be a Northern Ireland and that people and politicians in the Republic would have to look at what they had in common with those north of the border, e.g. tourism, transport and trade. In their view this was the only way to ever get political union.
Q. But that does not take account of the unionist mentality?
A. There has always been a very strong siege mentality among unionists. Those in Northern Ireland described themselves as Ulster Unionists as opposed to the Irish unionists in the Republic. The Ulster Unionists didn't trust London. The first NI Prime Minister James Craig was very suspicious of Lloyd George. He knew that Lloyd George just wanted rid of the unionists. I believe there is still a feeling among unionists that they cannot trust the British Government.
Q. Given the influential position of the DUP during the current Brexit negotiations, how do you see things playing out?
A. There is a question that has to be answered: who speaks for Northern Ireland in this debate? The people there voted to remain in the EU but the DUP were strongly for leaving. Does the DUP really speak for Northern Ireland at Westminster? We have to realise there is a lack of voice for a broad swathe of opinion in Northern Ireland.
There is quite a level of cross-community support for the backstop agreement and the DUP could be said to be on the wrong side of that argument. They have to remember that their position of influence at Westminster is fleeting.
Q. Is the insistence on the backstop a result of Ireland not trusting the UK to stick by agreements?
A. There are historical reasons for Irish politicians not to trust the UK. They remember promises made when partition was being discussed. They were led to believe that the Boundary Commission set up to examine the border would either get rid of it or make Northern Ireland so small that it would not be feasible. Lloyd George was speaking out of both sides of his mouth.
The insistence on the backstop is because the Irish feel they cannot simply take spoken guarantees.
Q. What do you make of Leo Varadkar's comments in December that nationalists will never again be left behind by Irish Governments?
A. Northern nationalists feel they have been abandoned by successive Irish Governments. In the 1950s, for example, they were refused permission to address the Dail about their concerns. That has led to distrust by northern nationalists.
Those people who live along the border on either side also have a distinct identity. They feel they don't quite belong to either jurisdiction. You also have to remember those who were left on the 'wrong' side of the border when partition occurred, such as Catholics in the north and Protestants in Monaghan, Cavan and Donegal.
Q. What is the attitude to Irish unity among the Republic's politicians?
A. It has changed but you have to wonder how deep the commitment to it is or whether it is just because the issue has been put centre stage by the Brexit debate. Fianna Fail and Fine Gael know that their titles indicate they want Irish unity but at the same time there is a nervousness about the prospect of taking on something which could be hugely problematic - the very substantial minority of people (in all-Ireland terms) who don't want unity. The parties may say they want unity but they talk about it as an abstract issue.
Sinn Fein, of course, are going about demanding a border poll but the other southern parties are being more cautious, saying that demand is not helpful at this time.
Q. What are your concerns about Brexit?
A. We have to remember that the border is not just about politics or even trade. It is about people's everyday lives, for example, sending children across the border to school in Northern Ireland or people who take a heart attack in Donegal being treated in Derry rather than travelling down to Sligo.
Q. Smuggling has been rife in border areas at various times. Your book has some entertaining anecdotes.
A. There is the tale of the old woman who carried a hot water bottle across the border telling customs it was needed to ease her rheumatism. In reality the bottle was full of whisky.
Then there were people who dressed up as customs or police officers and intercepted drovers smuggling cattle across the border. The drovers naturally fled and the bogus customs and police officers made off with their cattle.
Q. How do you explain that Protestants have left the Republic in large numbers since partition yet Catholic numbers in the north have increased to almost parity with Protestants?
A. It has to be accepted that Protestants felt the state after partition was a very alien place to them. By the 1920s Ireland was 93% Catholic.
At that stage Protestants still had a high proportion of jobs in the professions, but the exodus had begun before the War of Independence. However, it cannot be denied that there was intimidation and that the atmosphere for many was menacing.
But I don't go along with the idea that there was some sort of ethnic cleansing going on. To compare what happened in Ireland with what happened in other parts of Europe at that time would be a total exaggeration.
It is ironic that many southern Protestants felt abandoned by their Ulster Unionist kin. Even Edward Carson described himself as an Irish unionist.
Q. Do you think the current Brexit debate is harming the peace process?
A. I feel it has put the process back. One of the major planks of the Good Friday Agreement is that the UK is required to be totally impartial in relation to Northern Ireland. Some people feel that has been undermined by Theresa May's reliance on the DUP.
Tensions between north, south and Britain have been elevated and we are now hearing language which reflects pre-Good Friday Agreement mistrust.
You have to remember that of the 650 MPs at Westminster only 108 were elected before the Agreement was signed. There may be a lack of appreciation among the rest about why and how it came about.
The Border: The Legacy Of A Century Of Anglo-Irish Politics, by Diarmaid Ferriter, is published by Profile Books, price £12.99