The Good Friday Agreement may therefore have to be renegotiated during the Brexit negotiations, and any changes to the agreement required by Brexit or otherwise proposed can only be ratified by the Republic of Ireland in a new referendum. Ireland may have no option but to veto any post Brexit agreement between the UK and the EU if it cannot be ratified by referendum as otherwise any changes would be in breach of the Irish Constitution. Approval of any Brexit agreement may therefore be in the hands of the Irish people rather than something which can simply be negotiated with the Irish Government in some kind of back room deal.
It is doubtful that any Irish government could win a referendum on the future status of Northern Ireland which did not provide for a united Ireland within the EU at some point in the future - always subject to such a proposal also being approved by majority vote in Northern Ireland, as also provided for in the Good Friday Agreement. This means that a new referendum on a United Ireland could become an intrinsic part of the Brexit negotiations. Opinion polls in Northern Ireland have always shown significant majorities for remaining in the United Kingdom. Why should a refereendum held post Brexit be any different?
The most important factor influencing a change of sentiment would be if Scotland were to become independent. The vast majority of the "planters", or protestant settlers, who were settled in Northern Ireland following the defeat of the Irish chieftans there by Crown forces came from Scotland, not England or Wales. An independent Scotland would greatly reduce Northern Irish protestant family and historic ties with the rest of the UK.
Much has been made of more recent demographic changes in Northern Ireland whereby the Roman Catholic Nationalist population is growing much more rapidly than the protestant Unionist one. These changes have yet to produce significant changes in voting patterns in Northern Ireland however, and if anything has resulted in a larger non-aligned or dis-engaged adult population who do not necessarily wish to be defined by either the Catholic Nationalist nor the Protestant Unionist political identities.
But other things have changed. For instance the Pro-Brexit side led by the Democratic Unionist party was soundly defeated (56 - 44) in the UK Brexit referendum vote. This despite the fact that many nationalists may have had mixed feeling about the Brexit vote, knowing, on the one hand, that Brexit could re-introduce hated border controls, but on the other, that a Brexit vote might well precipitate a United Ireland referendum much sooner than if Britain remained within the EU. These mixed feelings may therefore explain the much lower turn-out in the Brexit referendum in nationalist areas.
But many other things have changed too. The Republic of Ireland has changed from being a much poorer agrarian state than Northern Ireland in the early 20th. Century, to being a much richer industrialised and post industrial state in the early 21st. Century. Clearly political independence has worked for the south much better than being an outlying de-industrialising region of a UK economy increasingly centralised around the financial services industry in London.
Furthermore, the south has moved away from being a theocratic Roman Catholic based polity which dominated a diminishing protestant minority to being a more liberal, diverse, secular and tolerant society where laws are made by popular mandate rather than by religious dogma. Far from feeling threatened by Roman Catholic domination, many socially conservative Protestants might wish to make common cause with the Catholic Church in support of more conservative social values.
There remains, however, one barrier to a possible united Ireland: The estimated £11 Billion net cost of maintaining the Northern Ireland state to the British Exchequer. It will be very difficult for the relatively small southern Irish economy to absorb that cost and very difficult for Northern Ireland unionists to believe that it could credibly do so. There might thus well be a widespread conviction, North and south, that whatever their personal political preferences, a United Ireland would simply not be financially viable and would result in a £11 Billion drop in living standards both North and South, at least in the short term.
However this is where the UK's strategic interest comes into the equation. Having baulked at the UK's net £10 Billion contribution to the EU for which it gets access to the Single Market, why should it continue to subsidise Northern Ireland to the tune of £11 Billion, for which it gets nothing but trouble in return? Teresa May may have stressed her allegiance to the Conservative and Unionist Party, but would her leadership survive a failure of the Brexit negotiations? Significantly she has visited Scotland, but not Northern Ireland in the immediate aftermath of her election.
The stage is therefor set for a grand bargain on Northern Ireland as part of the Brexit and post Brexit negotiations. A new referendum, North and south, would be held on the significant modification of the Good Friday Agreement required by the exit of the UK from the EU in any case. But in this new agreement, provision will be made for a much more precise definition of what entering a united Ireland would mean. If voters in Northern Ireland are offered a simple in-out choice, they will naturally take the safer status quo option. A United Ireland, so vaguely defined, simply feels like a takeover of the North by the south, and would, provoke fears of renewed civil unrest, if not outright civil war desired by nobody.
So the new Good Friday Agreement, agreed as part of the Brexit negotiations would provide for a much more nuanced choice: Continued Union with the UK, but with a gradually diminishing level of subsidy by the British exchequer, or a United Ireland, with a similar level of subsidy by the UK for a prolonged transitional period - say 10 years - but also a very gradual transition from current Northern Ireland political structures towards more integrated all Ireland ones, continued devolution of some powers to Northern Ireland (and perhaps other regions within Ireland, and continued membership of the EU and the guarantees on religious freedom and human rights contained in the Charter of fundamental rights of the European Union - yes, that same Charter which Teresa May has said she wants to abrogate in the UK.
Most voters, when faced with a choice between a concrete status quo and an unknown, doubtful and aspirational abstract choice will always choose the former, whatever their dissatisfaction with the status quo. However the United Ireland choice becomes more attractive if it seen as closer to the status quo: continued membership of the EU, roughly equivalent levels of financial subsidy, continued guarantees on human rights, and only gradual, measured and known changes to political structures which are seen as pretty dysfunctional anyway.
Ah but what when the British Exchequer subsidies run out, I hear you say? Well firstly, there is no guarantee that this will not happen regardless of what choice voters make. If the UK economy declines post Brexit, there may well be extreme pressures on UK public finances and political pressures to focus resources on declining regions of England and Wales which actually return Conservative or Labour MPs who might become part of a Government majority.
Secondly, there is no reason why an Irish government which has made a relative success of developing the Irish economy cannot make a greater success of developing the Northern Ireland economy, thus reducing the need for external subsidies. Irish Governmental policies have always been more proactive about developing the economy than free market ideologists in London centric administrations. Irish Diplomats have long commented on how British Government eyes tend to glaze over at any mention of Northern Ireland. At least the Irish government would have an incentive and would actively want to develop the Northern Ireland economy: The composition of future Irish governments would likely have a large Northern Ireland dimension in order to achieve a parliamentary majority.
Thirdly, the reduction in duplication of administrative overheads and increase in size of the domestic economy should also produce some administrative efficiencies and economies of scale, helping to reduce the overall public sector deficit in Ireland, North and South.
Fourthly, it is to be hoped that the EU, through re-energised social, regional, structural and cohesion programmes will make an ever greater contribution to the development of disadvantaged regions, partly because of the loss of the British neo-liberal economic influence within the EU, and perhaps as a formal part of the new Good Friday Agreement, negotiated as part of the Brexit agreement. That would provide greater certainty for all, and thus a greater likelihood of a positive outcome to any referendum.
There is a possible flaw in this argument however: It assumes that future decisions about Northern Ireland will be driven largely by economics - by a rational evaluation of the relative economic uncertainties and opportunities offered by a United Ireland rather than by a diminishing status quo. Politics isn't always rational, and more often it is driven by base anxieties and fears. The risk of a return to large scale violence will be a fear driving the decision making by many people. Best leave well alone might well be the reflex response.
Politics is also often driven by a sense of identity, one developed over many generations rather than on considerations of immediate or potential advantage. Do Northern Unionists feel so much part of a UK (possibly without Scotland) that it would override all other considerations? For some, that will undoubtedly be the case. Better to stick with your own rather than give nationalists something to crow about... But will such considerations still be the dominant motivation of the majority?
If there is one trend apparent in Northern Ireland, it is that an increasing number of people do not want to be painted in either Nationalist or Unionist colours, and will vote pragmatically for what seems like the prospect of a more prosperous future. It is up to Irish leaders, North and south, to give them a real prospect of such an option. Merely beating a tribal drum doesn't cut it anymore.
People in the south will also have to make a genuine effort to build more relationships with the North, and particularly with Northern Unionists. Many years of separation, sectarianism and the Troubles have created quite a social distance between both populations. Northern Unionists may be feeling increasingly isolated with respect to British politics (especially if Scotland becomes Independent), but that does not mean that they feel any closer to their southern neighbours.
There are few enough popular institutions which build such common feeling - the Irish Rugby Football Union being one - so if any United Ireland proposal is going to achieve majority support AND actually work well in practice, there is going to have to be a far greater civil society effort, North and south, to build those links which can bind communities together. A united Ireland can be facilitated by diplomacy, politics and economics, but only people working to build links across disparate communities can make it real. That is the real challenge which Brexit and its aftermath pose to all of us.