A High Court judge quashed a decision to stop a convicted terrorist murderer continuing to work as a groundskeeper.
Mr Justice Maguire also held that former Finance and Personnel Minister Peter Robinson breached the Ministerial Code in a case brought by west Belfast man Martin Neeson.
On Tuesday, he ruled that the Stormont department should now reconsider Neeson's suitability for a job he carried out for 19 years.
Lawyers for the 58-year-old ex-prisoner declared the verdict a "resounding exoneration" for him and others in his position.
He is also now expected to secure damages for his loss of employment. Neeson's judicial review challenge centred on an alleged failure to continue with guidance for employers on recruiting staff with conflict-related convictions.
He was released from jail in 1987 after serving eleven-and-a-half years for a murder committed when he was aged 16.
No further details of the killing four decades ago were disclosed in court. He has also convictions for belonging to a proscribed organisation and two counts of attempted murder.
Following his release he secured groundskeeper work with a conservation charity in the Poleglass area of Belfast.
But due to a technical change in the body's contractual relationship with the civil service he had to undergo security vetting.
Checks carried out by Access NI resulted in the Department of Finance and Personnel deciding in December 2014 that he is unsuitable to continue in the role.
Neeson went to Sinn Fein to seek their help in lobbying on his behalf. But after that proved unsuccessful he issued proceedings against the department. His legal team argued that he was unlawfully and irrationally deemed unsuitable for a job he had carried out for nearly 20 years.
They also contended that the move was based on an earlier decision taken by Mr Robinson during his tenure as Finance Minister back in 2007 that should have gone before the full Executive.
The court heard the guidance was formulated as part of the Good Friday and St Andrew's political agreements, and adopted by the Office of the First Minister and Deputy First Minister (OFMDFM) before Mr Robinson decided it should not apply.
It was claimed under Stormont rules the then Finance Minister should have taken the issue to the Executive because it was significant, controversial and cut across departments.
Counsel for Neeson said Sir Nigel Hamilton, a former head of the Northern Ireland Civil Service, appeared to be unaware the guidance had been revoked when he gave evidence to the Finance and Personnel Committee in 2012.
Ruling on the case today, Mr Justice Maguire said the decision to dis-apply the May 2007 guidance clearly related to a controversial and/or significant matter - the reintegration of former prisoners with conflict related convictions into the world of employment.
He held that Mr Robinson had breached the Ministerial Code because the matter should have been brought before the Executive Committee.
The judge added that he had no hesitation in concluding that the decision was one which no reasonable authority could lawfully have arrived at.
"On the facts of this case declaring the applicant unsuitable for employment flies in the face of being a rational decision," he said.
On that basis he quashed the decision and called on the Department to reconsider the matter - ideally by a senior official with no previous involvement in the case.
Outside court Neeson's solicitor, Niall Murphy of KRW Law, said: "This judgment is a resounding exoneration for the citizenship rights of the ex-political prisoner community.
"The decision made by the Civil Service to render Mr Neeson unsuitable to cut the grass in Poleglass has been declared irrational by the court."
Mr Murphy added: "That the judgment also declares that Peter Robinson breached the Ministerial Code is also a matter of grave concern."