This is the third time Hillsborough has been the site of a crucial meeting at which the fate of Northern Ireland has been at issue. In January 1974 I was part of an Irish government delegation that flew there by helicopter to meet members of the newly-established power-sharing executive.
The unionist members of that executive were by that time already under great pressure from extremists opposing the Sunning-dale Agreement, and that meeting proved to be the start of the process that led to the fall of that first Northern Ireland power-sharing government.
Then in November 1985 the Anglo-Irish Agreement was signed there by Margaret Thatcher and myself, while in the street outside unionist opponents of that agreement noisily demonstrated.
But last Thursday, in the same location, that 1985 agreement yielded its long-term fruit in the form of a joint declaration by the two governments. For this declaration could prove to be the penultimate stage of the peace process that was precipitated by the one-third decline in electoral support for Sinn Fein's Armalite-and-ballot-box strategy that followed the 1985 agreement.
The process of bringing a lasting peace to Northern Ireland has thus proved to be a long and tortuous one. We should not be too surprised at this. For the process of getting those who have been engaged in a terrorist campaign and those who have been the targets of that campaign to join in participation in government was bound to be complex and, above all, very slow.
In a first stage, those who had been the objects of 25 years of violence had to be persuaded to participate in indirect contacts with their enemies through intermediaries. Then they had to agree to sit in the same room without speaking to Sinn Fein. Next they had to engage in debate within that forum.
But only in recent weeks have they met privately for face-to-face talks. And only when that happened could any kind of mutual trust begin to develop between people who have for so long been divided by sectarian hatreds, and, in recent decades, persistent violence.
There is no substitute for such direct contact for, while these contacts do not necessarily produce trust and confidence between the participants, it is certain that without face-to-face discussion neither side can form a judgment about the other's real intentions.
Although this week's talks have still not produced agreement on ways to break the decommissioning deadlock, reports of these events leave no doubt about the change that has taken place in the personal relationship between Sinn Fein and UUP leaders, a relationship at Hillsborough that has even been described as "cordial".
While this in itself does not ensure agreement will be reached on the removal of the decommissioning obstacle, it offers much greater hope for the successful operation of the executive that will emerge if and when this problem is eventually resolved.
This is important, because the structure of the executive will differ from that of normal cabinet government in ways that are likely to test seriously the mutual trust and confidence of its members. For it does not seem that collective cabinet responsibility will apply to this executive in the manner to which we are accustomed in this and many other democratic states.
First of all, the members of the executive, and the posts they will hold, are not to be in the gift of the First Minister, or of the First and Deputy First Ministers jointly. Instead, both the nominations and the choice of posts seriatim will lie with the individual parties.
The First Minister and Deputy First Minister will thereafter "deal with and co-ordinate the work of the executive committee". The executive committee "will seek to agree each year, and review as necessary, a programme incorporating an agreed budget, linked to policies and programmes", a task that will require cross-community endorsement by the Assembly.
Finally the executive committee "will provide a forum for the discussion of, and agreement on, issues which cut across the responsibilities of two or more ministers, for prioritising executive and legislative proposals, and for recommending a common position where necessary".
NOW, phrases such as "seek to agree . . ." and "a forum for discussion of, and agreement on, issues" suggest a process a long way from the kind of decision-making system that applies in traditional cabinet government systems. Moreover, the phrase "issues which cut across the responsibilities of two or more ministers" seems to imply a considerable degree of individual rather than collective ministerial responsibility for those issues that fall exclusively within the competence of each minister.
Now that the leading members of the parties who will be thrown together in this new executive have started to talk directly, and intensively, to each other, this direct negotiating process may well bring about precisely the kind of mutual solidarity and trust that will be needed to make this unique governmental process work.
We seem now to be nearer than at any time in the past to seeing such an executive coming into existence. For the two prime ministers' joint declaration offers a new possibility of a way out of the apparently insoluble decommissioning dilemma.
The proposals in the declaration look like being acceptable to the UUP, although one should never underestimate the capacity of unionism to fracture as individuals succumb to pressures from its extreme wing, and as some members occasionally allow leadership pretensions to distract them from the pursuit of a common goal.
The Sinn Fein/IRA reaction to the declaration is more difficult to judge. Clearly, the Sinn Fein negotiators have been disconcerted by, and indeed visibly angered at, the Taoiseach's support for these proposals.
There has, of course, always been a danger that Sinn Fein would allow itself to be deceived by its own earlier propaganda about nationalist solidarity against unionism; failing to appreciate that any Irish government would have to take a more balanced position by holding the ring in some measure between the two sides in Northern Ireland.
Sinn Fein may, in particular, have relied too much on past perceptions of differences between Fianna Fail and other constitutional parties in the Republic on the Northern issue. The reality is that since the early 1970s these differences have been quite small, even during most of the periods during which Charles Haughey was Taoiseach.
In the public mind and in that of Sinn Fein these differences may have seemed greater than they actually were because of Fianna Fail's traditional republican rhetoric.
But since the 1920s that rhetoric has often been influenced as much by the need to bring and keep verbal republicans within the constitutional fold as by conviction.
In the end, Realpolitik dictated that some action on arms by the IRA would be required in order to get the devolved government under way in Northern Ireland. It is certain that without something of the kind Trimble would simply be unable to bring enough of his party with him to trigger this process.
That being so, it was inevitable that at the end of the day pressure should come on Sinn Fein and the IRA to make some move in this area.
Such a move has certainly been facilitated by the timetabling proposed in the declaration, by the idea of a Day of Reconciliation, and by the substitution of the phrase "put beyond use on a voluntary basis" for the hated decommissioning word.
Moreover, in a curious way the hands of Gerry Adams and Martin McGuinness visa-vis hardline colleagues may have been strengthened by this declaration. Hitherto, any effort they may have made to persuade other Army Council members of the need for flexibility were open to rejection on the basis that there remained a possibility of getting the executive going without any action on arms.
But now the choice is unequivocal: unless something along the lines now proposed is agreed, there simply will be no executive, and the peace process initiated by Sinn Fein will be at an end.
The moment of truth has arrived.