Edward Fitzgerald QC, for all five, argued they were wrongly denied their right to an immediate hearing before the Parole Board after completing 20 year jail tariffs imposed on them for "retribution and deterrence".
They were now facing extra months in prison because the Home Secretary, Michael Howard, did not refer their cases promptly to the board before the tariffs period expired in July, and because the board refused to review their cases as a matter of urgency, he said.
The five include Brendan Dowd, once named as top terrorist organiser in Britain, and Paul Norney, the longest serving Republican prisoner. The others are Noel Gibson, Sean Kinsella and Stephen Nordone.
Sixteen life sentences were passed on them at Manchester Crown Court in 1976 after they were found guilty on charges including conspiracy to murder, cause explosions and attempted murder. The trial judge described them as "dedicated and utterly callous terrorists".
Yesterday, Mr Fitzgerald told Mr Justice Dyson there was now overwhelming evidence that they were no longer dangerous. Given the Northern Ireland ceasefire and ongoing peace process, it was unlikely they would return to violence.
He said both European and domestic law gave them the right to be released unless they continued to pose a danger to the public.
There was a danger they were now being unlawfully detained, he said. The five had been told their cases could not be considered until December because of the long queue of applications before the board and they would not be allowed to jump the queue.
In a case which could affect many other discretionary lifers, Mr Fitzgerald asked the court to declare the five were entitled to an immediate hearing.
Kenneth Parker QC, for the Home Secretary and the Parole Board, argued that the minister had a discretion whether to refer cases to the board and had not infringed either domestic or European law.
"What Mr Fitzgerald is saying is that every person (discretionary lifer) has a statutory right to have his case referred to the Parole Board, either immediately or soon after the tariff has expired. If Parliament had intended that, it would have said so," he said.