Where does joint co-operation end and joint authority begin?
What role did the Taoiseach Brian Cowen and his Foreign Minister Michael Martin play in the Hillsborough talks?
I ask that question because the Good Friday Agreement was based on three distinct 'strands' of relationships.
Strand one focused on internal matters within Northern Ireland as part of the UK and created the Assembly and Executive.
Strand two covered relations between Northern Ireland and the Republic of Ireland. It established a North-South Ministerial Conference bringing together members of the Northern Ireland Executive and the Irish government to oversee six cross-border implementation bodies.
Strand three dealt with the east-west relationships within the British Isles. A British-Irish inter-governmental conference was established to promote bilateral co-operation between the UK and Ireland.
Stormont's internal strand one powers embrace those matters devolved from Westminster, ranging from health to education, the environment to regional development and the right to raise a local budget.
Responsibility for policing and justice, though not yet transferred from Westminster to Stormont, should fall into the strand one category. But the question raised by the Irish presence throughout the Hillsborough talks is: what is and what is not a solely domestic matter for the UK Government and Northern Ireland Executive?
The Good Friday negotiations were structured meticulously to take account of unionist sensitivities. The big fear was that unionists would not sign up if there was any suspicion of interference by the south in Northern Ireland's domestic devolved administration.
The creations of three separate 'strands' was a masterstroke, enabling the unionists to feel collectively comforted by strand one and strand three and the nationalists and republicans to have their place in the sun through strand two's north-south links.
The more disputes arise over what has, or has not, been implemented in the original three strands of the 1998 accord, the more London and Dublin governments appear to act as one. This closeness was apparent at St Andrews in 2006 and it was copper-fastened as never before at Hillsborough this year.
In regard to strand one, there are two specific references in the St Andrews agreement to the British Government acting in "consultation as appropriate with the Irish government" to affect legislation "outside the scope of the devolved institutions".
John Major's relationship with Albert Reynolds, and subsequently Tony Blair's with Bertie Ahern, offered Dublin an irreversible interest in Northern Ireland affairs culminating in the Downing St accord of 1994, the Good Friday Agreement and St Andrews.
Even the DUP itself has come light years from that wintry afternoon in the 1960s when Ian Paisley pelted Sean Lemass's car with snowballs in protest against the very first visit by a southern leader to Stormont.
The southern toe in the northern door first appeared in 1985 when Margaret Thatcher and Garret FitzGerald signed the Anglo-Irish Agreement at Hillsborough.
Of course, Thatcher stressed to unionists it was an arms-length arrangement. FitzGerald, on the other hand, as I well recall over a dinner in his Dun Laoghaire club at that time, argued that he had won more than mere consultation. He interpreted the Anglo-Irish agreement as giving Dublin a strong, influential say in Northern Ireland which could be built upon over the years.
The First Minister Peter Robinson, in a recent interview, drew a comparison between Hillsborough 1985 when unionists stood outside in protest and 2010 when they sat at the negotiating table.
What he didn't say was that British consultation with Dublin 25 years ago was cursory compared to the Irish presence in 2010.
The rules of engagement of the Irish government at Hillsborough are unclear. However, the photo-call images of plenary sessions involving all the parties show the British and Irish leaders side by side, Gordon Brown with Brian Cowen, Shaun Woodward with Michael Martin. The overall impression of Hillsborough is of two governments acting as one. It can be argued that the Irish presence throughout the negotiations was a recognition of the complexity of the policing and justice issue; that the Irish government had as much right as anyone to be there given it was a co-signator of the St Andrews agreement which was not being honoured; and that policing and justice was only part of a much wider negotiation involving many issues in which Dublin had a legitimate interest dating back to the Good Friday accord.
On the other hand, policing and justice does appear to be a strand one responsibility in so far as it requires the transfer of political responsibility from one part of the United Kingdom to another.
Many doubts remain over the Hillsborough deal. If the deal fails, it will be left to the two governments to pick up the pieces. In that event, plan B envisages an enhanced level of co-operation between London and Dublin in overseeing Northern Ireland.
For the time being, the manner in which the Hillsborough negotiations were conducted suggests joint co-operation, as distinct from consultation between London and Dublin on Northern Ireland matters, is now standard practice.
That being the case, Hillsborough poses the question: where does joint co-operation end and where does joint authority begin?