In that time Britain moved, in the Irish psyche, from an ancient enemy still to be treated with suspicion to a neighbour with shared interests.

By 1989, when Chief Superintendent Harry Breen and Superintendent Robert Buchanan were murdered at Jonesboro, the process was fairly advanced.

The Smithwick Tribunal in Dublin has heard that the officers died returning from an intelligence exchange with gardai where a joint operation against Thomas 'Slab' Murphy, chief of staff of the IRA, was planned. That showed progress.

Yet the fact that the officers' movements were compromised led to suspicion that some in the gardai may have helped the IRA target them. At the tribunal, three retired officers all denied involvement.

It is pointless pre-judging the inquiry, but at the time security forces north and south dismissed collusion allegations.

Brian Fitzsimons, head of RUC Special Branch, told me he did not believe gardai were involved. He also denied claims that the IRA had taken Mr Breen's notebook containing intelligence details: "We have the chief superintendent's briefcase and papers," he said.

He blamed a security lapse by Mr Buchanan, who used his own car instead of a pool vehicle and parked it in front of the Garda station.

Was Mr Fitzsimons telling me the truth? Or was he playing a larger game to keep the lid on things and help preserve cross-border relations?

At this stage, extradition of terrorist suspects, long impossible, was starting to happen. Was developing such co-operation of greater strategic importance than the satisfaction of pointing the finger for a single atrocity?

Did he intend to 'turn a trick', in Special Branch parlance, by getting information from a rogue garda with the help of his colleagues?

Certainly, the Irish state's relationship to the IRA was murky and ambiguous when the conflict ignited.

Unable to directly help northern nationalists under pressure, it set up refugee centres, trained northern civilians in the use of arms and even offered guns and money to insurgent groups, much as Britain is threatening to do in Syria today.

John Kelly, an IRA man and later a Sinn Fein MLA, told me in 2005 of meetings with ministers: "Initially, our contacts were with the Irish government, as we understood it. We met Brian Lenihan and Paddy Hillary, Jack Lynch and Jim Gibbons [the defence minister] - there were others."

Kelly added: "I said 'There is no need for blankets or feeding bottles. We need arms to defend the people'. They accepted all that. It was open, transparent and above board. There was no subterfuge, no winking and nodding and no cute hoorism."

In the subsequent trial, the men he named did not feature and everyone was acquitted, although, as John Kelly told me before his death, the plot was real enough.

It was an age away from official receptions for the Queen. Mr Justice Smithwick will have to decide what point in the spectrum had been reached in March 1989 when the IRA opened fire in Jonesboro.