The remains of Captain Robert Nairac most likely lie in Ravensdale Forest, Co Louth, the author of a book on the undercover British soldier claims.
Alistair Kerr says Nairac was buried within a 10-mile radius of the field in Ravensdale where he was shot dead by IRA members in May 1977.
This confirms assertions by republicans and the Independent Commission for the Location of Victims Remains that his body wasn't put through a meat processor.
The book raises the possibility that everyone who knew the precise location of the burial site may now be dead.
But it expresses hope that former IRA members gave Boston College researchers information on Nairac's murder and secret burial, which will be disclosed when the oral history archives are eventually opened.
Mr Kerr, a retired British Foreign Office diplomat, says he set out to separate myths from facts in his book Betrayal: The Murder Of Robert Nairac.
He believes it's important to "disentangle the reality of Nairac from the dense thicket of legend and published disinformation" surrounding him.
Mr Kerr says material given to him "exonerates" Nairac from allegations of involvement in murder, including that of IRA member John Francis Green, who was shot dead in Co Monaghan in 1975. "Two reliable sources have shown that he was in Londonderry on that night and that he had been based there for some time before Green's murder," Mr Kerr said.
Earlier this year the Green family said they believed Nairac was involved in the killing, although they bore him no ill will and called for the return of his body.
The book is heavily critical of Colonel Clive Fairweather, an SAS officer in the 1970s who went on to become Scotland's chief inspector of prisons.
It claims he made fatal delays following Nairac's abduction from the car park of the Three Steps Inn in Dromintee, where he had been posing as 'Danny McErlean' from Belfast as he drank with locals.
It is alleged that Colonel Fairweather had loathed Nairac, believing him to be a loose cannon who naively and recklessly went on unauthorised intelligence missions and put others' lives in danger. Fairweather died five years ago, but his friends have strongly denied the claims made about him.
The book addresses claims that Nairac was gay and had an Army lover, but says there is a lack of evidence to support the rumours.
"Unless a credible witness comes out of the shadows and says: 'You are completely wrong, I was Robert Nairac's lover and a good time was had by us both', we shall never know," Mr Kerr said.
The author describes Nairac as "generous, highly principled, deeply religious, outstandingly brave and above all a fantastic friend."
He describes the undercover soldier as a deeply conservative Catholic.
He says assaults "as vicious as those inflicted on his living body" have been made about his character.
"Republicans had a role in this but others who were at the heart of British politics and former Army officers also played a part," the book states.
Mr Kerr added: "If Nairac had been alive now, he would be white-haired. He would be 69 years old.
"This comes as a slight shock since he remains in our collective memory as a vigorous young man."