The agreement reached this week in Belfast is the culmination of a long and complex process. In a sense, it could be said to have begun almost 40 years ago when Donal Barrington - then a young lawyer, now a Supreme Court judge - published the text of an address he had given to Tuairim. (Tuairim was a forum for debate, created by a number of young people who were moved to challenge the intellectual, political and economic stagnation and sterility of Irish society at the end of the first generation of Irish independence).
In that pamphlet, Donal Barrington challenged the irredentist approach to Northern Ireland that had become the staple diet of political rhetoric in this State - the claim that Northern Ireland simply had no right to exist and that Britain should hand it over to us regardless of the wishes of a majority of its population.
Those of us who shared Donal Barrington's view were a small and unpopular minority then - and for a long time afterwards. It was not until September 1969 that, with Paddy Harte's assistance, I managed to get Fine Gael to commit itself in a policy document to the principle of consent, as Conor Cruise O'Brien succeeded in doing with the Labour Party around the same time. And in both parties several years were to elapse before the principle of consent as a precondition for any political unification of the island became fully and whole-heartedly accepted.
Within Fianna Fail this process was much more long drawn out. While it had been evident since the Arms Crisis that Jack Lynch personally accepted the consent principle, both he and those in Fianna Fail who shared his view were deeply conscious of the crucial "mudguard" role of Fianna Fail viz-aviz militant republicanism - the need to keep it a broad enough church to encompass a republican wing that might otherwise drift towards supporting the IRA.
De Valera's great achievement had been to bring back into constitutional politics a very large number of people who had been alienated from the new State at the time of the Civil War - and to keep them there. Therein lay the key to his policy of war-time neutrality, for if he had led Ireland into the war against the Nazis - as at the time many people like myself passionately believed should happen - there would have been a real danger that many of his followers would swing back to militant republicanism.
That could have led to a recrudescence of the Civil War - this time waged between a German-supported IRA with considerable popular support, especially from Fianna Fail dissidents, and the constitutional parties, Fianna Fail, Fine Gael and Labour. Fear of such a development was, I believe, the factor that led even the most pro-Allied politicians, like my father, to support neutrality - for no politician in any party was prepared to risk a repetition of the events of 1922-23.
So long as any significant residual potential of support for the IRA continued to exist in our State, the leaders of Fianna Fail saw merit in maintaining a constructive ambiguity on the consent issue, leaving it to Fine Gael and Labour to lead public opinion towards an explicit rejection of irredentism that would promote public support for the consent principle.
I have always believed that Fianna Fail's ambiguity on this issue was not motivated solely by a desire to maximise their vote but reflected a genuine belief at the level of the party's leadership - albeit somewhat differently presented by Jack Lynch and Charles Haughey - that the national interest was best served by this approach.
By 1993, however, the situation had altered considerably. Public opinion on the principle of consent had fully evolved - to the point where popular support for consent as a pre-condition for political unification was just as overwhelming as rejection of that concept had been a quarter-of-acentury earlier. The mudguard was no longer needed.
In the second place, a new Fianna Fail leader, Albert Reynolds had come to power. He carried no baggage from the earlier period of constructive ambiguity on this issue, and he could assert the principle of consent without being accused of contradicting himself.
Third, and most important of all, an evolution of opinion within Sinn Fein/IRA, encouraged by John Hume in private discussions with Gerry Adams at various stages in the late 1980s and early 1990s, had created the possibility of a movement towards peace. However, to help Sinn Fein/IRA to end their violence it was necessary to persuade the British government to enunciate its freedom from any vested interest in the Union and its willingness to have the future of Ireland decided by its own inhabitants - subject only to the principle of consent.
The price Albert Reynolds had to pay to secure the required British statement of disinterest in relation to the Union was unambiguous acceptance of the consent principle. The balance of advantage in relation to an unqualified - as distinct from a tacit but ambiguous - acceptance of this principle by Fianna Fail had finally and decisively shifted. Hence the Downing Street Declaration.
There is, of course, a deep irony in the fact that it was Sinn Fein/IRA who thus precipitated Fianna Fail's formal acceptance of the consent principle - to which clearly Sinn Fein/IRA itself remains unprepared to subscribe. But history is full of such ironies.
From a unionist point of view the unambiguous character of the reference to consent in the Downing Street Declaration was of crucial importance. It is true that it took time for its significance to percolate through the unionist community. Over many decades unionist attitudes towards our State had been frozen by a sense of being a beleaguered minority in the island of Ireland, and by the continuing impact of past hostile irredentist propaganda emanating from this State.
To a considerable degree the positive impact upon unionist party politicians of that Downing Street Declaration, as well as of the IRA cessation of violence, has been indirect rather than direct - mediated through non-political channels such as business and the churches. It took time for the influence of these external channels of opinion to outweigh the negative reactions of UUP activists and the fear of loss of support to other unionist parties.
By the middle of this year the message had got through - and a new sense of the need to move towards an agreement on the future of Northern Ireland had been generated by the emergence in Britain of a new government with an overwhelming majority in parliament.
The outcome of these talks, now finally about to begin, is inherently difficult to predict. The unionists will clearly want to be able to present any agreement reached as securing the Union - which they will, of course, be able to do by pointing to the commitment of the Irish Government and the SDLP to the principle of consent. That will ensure that the union will endure until and unless a majority of the people of Northern Ireland decide otherwise.
What of Sinn Fein/IRA? Sinn Fein has said it will stand by whatever is agreed in these talks. But it is not clear that the word "agreed" has same meaning for them as for the other parties, who understand it to mean agreed by consensus - in practice, agreed by the UUP and the SDLP.
However, the two governments have adopted John Hume's proposal that any agreement reached by consensus be submitted for approval by the electorate of the whole island on the same day. And this provides a vehicle by which Sinn Fein/IRA could regard a decision thus endorsed as being an act of national self-determination - even if Sinn Fein had not itself endorsed the agreement reached, or perhaps had actually opposed it in the talks. They could do so because, for the first time since 1918, the Irish people would have voted on the same day on a single proposal as to how the island should be structured politically - in this instance through separate administrations operating certain functions jointly.
In other words, a possible outcome of the talks - and presumably that to which the two governments are now directing their efforts - would be an agreement which was, perhaps, opposed by Sinn Fein, (as well as by the DUP and UK Unionists) but which was nevertheless endorsed by the Irish people North and South, thereby securing that Sinn Fein/IRA's dissent would thenceforth be non-violent.
There must, of course, be considerable uncertainty about whether the UUP and the SDLP can find agreement on such a settlement, to be submitted for endorsement by the people of the island, North and South, and also about whether such an agreement could secure such support in the North as well as in the South. But it is at least possible now to see for the first time a way in which the violence in the North might conceivably be brought to an end.
That is progress indeed.