Papers released by the National Archives under the 30-year rule reveal how the Attorney General’s office, justice chiefs and the Taoiseach’s department felt sure he would see out four decades behind bars.

The jail term was imposed after then president Patrick Hillery signed an order on December 20, 1985, which commuted McHugh’s death sentence to 40 years in prison.

McHugh had been due to be hanged seven days later.

On June 27, 1985, he and accomplice Noel Callan stole IR£25,288 from the labour exchange in Moorhall, Co Louth, at gunpoint, killing Garda Sergeant Patrick Morrissey in their getaway.

After crashing a motorbike near Rathbrist, McHugh and Callan attempted to flee on foot with McHugh firing at the unarmed officer, wounding him in the leg. He then lent over him and shot him in the face killing him.

Briefing notes for the Government on plans to avoid a hanging revealed McHugh, from Crossmaglen, Co Armagh, had regularly missed school, and was a handful for his adoptive parents.

“McHugh is regarded as being highly intelligent and reckless in the extreme when committing crime,” one note said.

In documents released to the National Archives, Government minutes from December 19 that year, just over two weeks after the death penalty was imposed by the Special Criminal Court, show how officials believed there was an “understanding” McHugh would never enjoy standard one quarter remission.

While this was revealed when Callan successfully sued the State in 2013 to get the remission, it had not been publicly explained at the time.

Government secretary Dermot Nally confirmed the decision in a letter to the Department of Justice.

“I am to inform you that the decision of the Government to advise the President to commute the sentence was arrived at on the understanding that the full sentence of 40 years would be served without remission,” he said.

The Government press statement at the time made no mention of the “understanding”, only that the 40 years would be served in full.

Other papers on the case revealed the Government felt sure it had the law on its side.

A legal officer in the Attorney General’s office, Mr Russell, offered no advice other than suggesting to Mr Nally that everything “appeared to him to be straightforward”.

The Government thought there was a precedent for its stance citing four other cases of Garda killings and advice from the Attorney General in 1981.

The ban on remission for Callan was dismissed by the Supreme Court in 2013, clearing the way for both men to be freed after 30 years on December 1.