The UK has refused to release archive material on paid Irish police informants from more than 100 years ago - amid fears that their descendants could be shunned or even targeted by dissident republicans.
Irish historian Barry Keane argued that the 100-year-old file may hold vital clues to such critical events in Ireland as the Phoenix Park murders of 1882 and precisely what Britain knew about plans for a rising in the years before 1916.
"I believe this file is absolutely critical to our accurate knowledge of major events in Irish history and there is clearly no reason why it cannot be released," he told the Irish Independent.
"It is patently obvious that anyone mentioned in this file is now long since deceased."
However, a UK Freedom of Information tribunal rejected Mr Keane's appeal against the refusal by the British Home Office and the Metropolitan Police to release the files.
Mr Keane said he was shocked when evidence was offered by a UK counter-terrorism expert from behind a screen, despite the fact that the last surviving subject in the file died over 60 years ago.
His appeal was later rejected a tribunal by a two-to-one majority.
The reasons cited by the tribunal for rejecting the application included the damage to the UK's ability to recruit informants and exposing their Irish descendants to boycott or even possible targeting by dissident republican elements.
The majority "believed it is by no means fanciful to suggest that on revelation that a person's ancestor was an informer, elements of the local community might choose to shun him or her, causing them distress".
However, the tribunal minority accepted all of Mr Keane's arguments that not to release information of such antiquity "simply fails a very basic common-sense test".
Mr Keane had told the tribunal that it was ridiculous to suggest that members of the Provisional IRA, Real IRA, Continuity IRA and other versions of the IRA would act on information more than 100 years old against the descendants of people long since dead.
Mr Keane and barrister Brian Leahy launched the challenge at a special UK information tribunal in London.
The Irish duo specifically asked the commissioners to direct the British Home Office, the Metropolitan Police and the UK Information Commissioners to release the full contents of the file, which was discovered in 2013.
The file was entitled 'Paid Informants in Irish Secret Societies 1886-1910' and was unearthed by the Cork historian during research two years ago in the UK National Archive.
Mr Keane, who was doing research at the time for his book on the Protestant massacres in west Cork during the War of Independence, was shocked to discover more than half the documents it listed were still withheld by the British Home Office. This was despite the fact that some records were 127 years old.