Chief Superintendent Harry Breen and Superintendent Bob Buchanan were gunned down on March 20 1989 near the border in south Armagh shortly after a meeting with a senior gardai in Dundalk, Co Louth.
A tribunal into collusion between the Republic's Garda and the Provos has found an unidentified IRA mole leaked information about their movements on the day they were killed.
In his 500-page report, Judge Peter Smithwick found there was collusion in the killings but he has not pointed the finger at any individual officer or civilian.
"It is particularly regrettable that both police services acted swiftly to dismiss speculation of the possibility of collusion rather than to deal with that by means of a thorough and credible investigation," he found.
"This was an example of the prioritisation of political expediency in the short term, without due regard to the rights of victims and the importance of placing justice at the centre of any policing system."
Judge Smithwick issued a series of damning findings on the culture and attitudes towards dealing with rogue police.
"The integrity of and confidence in An Garda Siochana can properly be maintained only if suggestions of inappropriate or illegal conduct by members are taken seriously, transparently and thoroughly investigated and, above all, not tolerated or ignored on the basis of some misguided sense of loyalty to the force or to its members," he said.
Mr Breen and Mr Buchanan travelled to Dundalk to discuss a possible joint RUC/Garda operation on lands owned by prominent republican Thomas "Slab" Murphy.
On their way home on the Edenappa Road near Jonesborough, just a few hundred yards over the border, a van with a gang of up to six armed IRA men cut them off.
Both policemen were in civilian clothes and neither was carrying a weapon.
Mr Buchanan tried to reverse the red Vauxhall Cavalier out of danger but got stuck in a ditch and was shot several times in the driver's seat.
Mr Breen tried to surrender. He got out of the car, walked to the front of it with his hands up and was shot several times.
The tribunal was established in 2005, with lawyers spending six years examining intelligence and witness statements from police, undercover agents, Provo bombers and politicians during 133 days of public hearings.
Three former garda officers - Owen Corrigan, Leo Colton and Finbarr Hickey - were granted legal representation and all vigorously denied allegations of collusion.
The tribunal was set up as part of a peace process deal after Canadian judge Peter Cory recommended a public inquiry be held into allegations of collusion by garda officers, or a civilian in the force, over the murders over Mr Breen and Mr Buchanan.
The tribunal found that Owen Corrigan - a former Garda detective alleged in the House of Commons, under parliamentary privilege, to have colluded with Provos in the murders - had an inappropriate relationship with subversives.
Judge Smithwick said Corrigan became disaffected, possibly in the 1980s, with the detective branch in Dundalk where he was based.
"I also find that what may have started out as a professional relationship with subversives for the legitimate purpose of intelligence-gathering ultimately developed into a relationship of an inappropriate nature," he said.
Corrigan's evidence to the tribunal was vague, evasive and inconsistent and was not credible, the report concluded.
"I do not think that he has been truthful to the tribunal in his evidence in relation to a number of matters," Judge Smithcwick said.
Furthermore, it was found that widespread concerns about Corrigan's "extra-curricular activities" - including smuggling and his relationship with subversives - were ignored by Garda chiefs.
"I believe that there were sufficient warning signs, such that senior Garda officers should have taken steps to have Detective Sergeant Corrigan transferred away from the border area earlier than this, in fact, occurred," Judge Smithwick said.
RUC concerns about him were relayed to Garda assistant commissioner Eugene Crowley - who went on to become Garda commissioner - in 1987 "but there is no evidence of any action having been taken on foot of this information," the tribunal found.
Ireland's Foreign Affairs Minister Eamon Gilmore apologised to the families of the dead RUC officers and said the country must confront the grave findings of collusion.
"I am appalled and saddened by this finding. It is a matter of grave public concern," he said.
"On behalf of the Government and the people of Ireland, I apologise without reservation to the Breen and Buchanan families for any failings identified in the report on the part of the state or any of its agencies."
Justice Minister Alan Shatter also apologised.
"Even with the passage of 24 years and the positive developments which have taken place on the island since, our condemnation of their murder should be as strong today as it was then," he said.
Mr Shatter said nothing in the report should detract from the good work of An Garda Siochana during the Troubles.
Judge Smithwick concluded his long-awaited and shocking expose with a devastating indictment of the abuse of power in some aspects of Irish life.
"The culture of failing adequately to address suggestions of wrongdoing, either for reasons of political expediency or by virtue of misguided loyalty, has been a feature of life in this state," he said.
"Too often that culture has resulted, some years later, after doubts, grievances and injustices have festered, in the setting up of investigations, commissions or tribunals of inquiry.
"This tribunal has sought to establish the truth and, in so doing, I hope that it has contributed one small part to changing that culture."
Garda Commissioner Martin Callinan said it would be inappropriate to comment without fully examining the report.
Northern Ireland's Justice Minister David Ford said: "A rogue officer acting outside the law should not result in the loss of trust between the PSNI (Police Service of Northern Ireland) and An Garda Siochana."
Theresa Villiers, Secretary of State for Northern Ireland, said she will raise the collusion scandal with the Irish Government.
"An important point to remember is that levels of co-operation between An Garda Siochana and the PSNI are now at unprecedented levels and are playing a crucial part in combating terrorist attacks in Northern Ireland," she said.
Tom Elliott, Ulster Unionist MLA, said the tribunal is confirmation that successive Irish governments did not do enough to stop the IRA.
"The Irish Minister for Foreign Affairs said in a recent speech in Cambridge that the Irish Government had to address the perception among unionists that successive Irish governments did not do enough to stop the IRA. We welcomed that statement as a useful first step," he said.
"Judge Smithwick's confirmation that it is more than a perception will require the Taoiseach to take the next step to address unionist concerns."
Democratic Unionist MP Jeffrey Donaldson said the findings of the report had major implications for the Haass talks process aimed at resolving Northern Ireland's difficult issues.
"I believe that the finding of collusion on the part of the Garda with members of the IRA is significant and has major implications for the way in which we deal with the past under the current talks process being overseen by Dr Richard Haass," he said.
"While the report does not identify those Garda officers responsible, those people know who they are and if they do not face justice in this world they certainly will face it thereafter."
Mr Donaldson said that any process to deal with Northern Ireland's troubled past would have to be victim-centred.