For years we have heard how closely the police forces on both sides of the border work together to bring people to justice.
We have heard it from politicians and we have heard it from senior officers.
At the Smithwick Tribunal, however, a different picture has emerged.
The Smithwick Tribunal has shown that trust is in short supply, writes RTÉ's Richard Dowling.
What we have seen in fact is how trust between parts of the two forces is in very short supply.
The two most senior intelligence officers on the island have now given direct evidence at the tribunal, which is investigating allegations that a garda assisted the IRA in murdering two senior RUC officers in March 1989.
In essence, the PSNI said they have current intelligence that claims there was garda collusion in a number of murders and officers assisted the IRA during the Troubles.
However, they refused to give their counterparts in the gardaí or the tribunal any further details other than the one or two line summaries that were made public at Smithwick.
We can't tell you anymore ... got to protect our sources ... trust us, they said. We've checked it through and it is accurate.
The gardaí on the other hand say they need more information to see whether they can believe it. Trust us, they said, we'll be careful with this intelligence ... we exchange sensitive material all the time, what's different now?
Neither side, it seems, is willing to trust the other.
The evidence of Assistant Chief Constable Drew Harris was that the intelligence they had obtained was "reliable and accurate" – the garda chief said his counterpart's assessment was full of "glaring omissions".
It is an unedifying sight, especially as this tribunal is trying to establish if there was collusion between gardaí and the Provisional IRA in the murder of the two most senior RUC officers to die during the Troubles.
For the families of Chief Supt Harry Breen and Supt Bob Buchanan, the evidence has proved to be particularly disturbing.
Both John McBurney for the Breen family and Ernie Waterworth for the Buchanan family have expressed their deep concern.
They have urged both police forces to work together in the search for truth.
The problem, however, is that neither police force is willing to take their counterpart's word on this issue.
It is possible, as senior counsel Mary Laverty pointed out, that British Security Service may have a role in this.
Everything that the PSNI have brought to Smithwick in terms of intelligence material first went through MI5.
It was they who decided what should or should not be released to the tribunal.
It is also important to remember that this is not a spat of inconsequence.
These two men are in charge of their intelligence sections within their own police forces and they are deeply divided.
No doubt we will hear it is all unlikely to impact on day-to-day co-operation between the PSNI and An Garda Síochána and hopefully it will not.
Mutual trust, however, is essential for police forces to work efficiently together. It is clear after this evidence that is sadly lacking.