UTV Live Tonight has examined RUC files on the atrocity which killed a total of 18 British soldiers, including 16 paratroopers, and was widely regarded as the IRA's bid for revenge for Bloody Sunday.
Despite damning evidence against two suspects - known IRA men Brendan Burns and Joseph Brennan, both from Crossmaglen - no one ever stood trial over the double bomb attack.
The pair were arrested minutes after the second blast in Co Down on 27 August 1979 and evidence against them included firearms residues, traces of ammonium nitrate, ferns in their clothing which could have come from the look-out sites and tests on saliva from a cigarette butt recovered from a lookout site.
But just two days later, the men were charged only with motoring offences and released.
"In spite of requests by us, we were not permitted facilities to interview the suspects - nor would Garda re-interview Brennan and Burns for us, using their equivalent of our crime squad," an RUC report from the time stated, outlining frustrations over the handling of the case.
A year later, an RUC detective chief inspector still held that politics had been a factor in jeopardising the police investigation.
It is unfortunately true that the Republic is a haven for terrorists on the run after the commission of outrages in Northern Ireland.
The tensions between the RUC and An Garda Síochána were highlighted decades later during the Smithwick tribunal into the murders of RUC men Bob Buchanan and Harry Breen.
The inquiry heard claims that gardaí were acting on orders straight from the top - Jack Lynch was Taoiseach at the time, succeeded in 1980 by Charles Haughey.
In 1981, efforts by the RUC to extradite Burns to Northern Ireland were thwarted and he accidentally blew himself up in 1988.
Brennan came to the attention of security forces again and was jailed for 16 years in 1994 for his part in an IRA murder bid.
He was released from the Maze after five years under the terms of the Good Friday Agreement.
"I am satisfied that, perhaps for political considerations, these 18 soldiers killed at Narrow Water Castle did not get justice from anyone - north or south," Kevin Sheehy, a senior RUC officer at the time, told UTV Live Tonight.
"Why? I think down south, there was interference at a very high level into the local Garda investigation. In Northern Ireland, I suspect that perhaps because of the peace process - where they were trying to get the IRA onboard - that it was maybe inconvenient."
The day of the Narrow Water massacre was one of the darkest in Northern Ireland's troubled past.
That morning, the Queen's cousin Lord Mountbatton was killed in an IRA bomb attack while fishing off the coast of Sligo. His young grandson, his daughter's mother-in-law and a local boy also died.
The detonation of two large bombs at Narrow Water, near Warrenpoint, came within hours.
It remains the biggest single loss of life for the British Army in Northern Ireland.
"If there had been civilians, there would have been a public outcry to get someone for it," Linda Kelly, the sister of one of the murdered paratroopers - 23-year-old Thomas Vance from Sandy Row in South Belfast - told UTV Live Tonight.
"He was 23. That was it, life over. We deserve justice and closure for him."
More than 30 years later, the Historical Enquiries Team is re-examining the Narrow Water case in the hope of finally delivering the truth about what happened.