Drogheda crime gang boss Cornelius Price has claimed he was kept in prison for 24 hours longer than necessary.
He has now launched an appeal against a court ruling that upheld how the governor of Wheatfield Prison calculated his release date on a three-year jail sentence.
The legal challenge centres on whether the Irish Prison Service should use days or months to fix the date for a prisoner's release from sentence that includes a fraction of a year.
Lawyers for Price (38) of Rockleigh House, Richardstown, Gormanstown, Co Meath, claim he was held for 24 hours longer than he should have been by the IPS.
Price was convicted at the Dublin Circuit Criminal Court in February 2017 on a count of recklessly engaging in conduct that created a substantial risk of death or harm.
The charge followed an incident at Balbriggan Garda Station on January 22, 2014, when Price drove a van at high speed at Garda Gary Dillane, forcing him to jump out of its path.
Yesterday the Court of Appeal heard how Price, whose sentence began on February 17, 2017, was released from custody on May 18, 2019, after being entitled to a quarter remission off his full sentence.
Price says he was told by the prison governor he was entitled to be freed on May 18, 2019, on the basis of serving three-quarters of his full sentence, calculated by days, once remission was taken into account.
But the father of two maintains he should have been released on May 17, 2019, based on an alternative method of using months to calculate a fraction of a year and where he would have served 27 months in prison.
Paul Carroll SC, for Price, said the current position of how the IPS calculated release dates was "very unsatisfactory" and not based on any statutory provision, rules or written policy.
Mr Carroll questioned the right of the IPS to operate what is "effectively a convention".
He claimed an alternative method of calculating release dates based on months was "equally valid".
Because of the IPS's method, Mr Carroll said Price should have been released earlier.
"It's not good enough when we are talking about someone's liberty," said Mr Carroll.
He told the court the issue had only been raised after Price's release date was published in the Sunday World.
Mr Carroll said that where calculations arrived at different results then a prisoner should be entitled to avail of the earlier release date.
In May 2019 the High Court dismissed Price's challenge as it would create uncertainty and potential injustice if the length of a sentence varied depending on the time of year it was served.
Lorcan Staines SC, for the governor of Wheatfield Prison, said the most numerically accurate way for calculating a release date as a fraction of a year was by days.
Mr Staines told judges the approach taken by Price represented an "inferior method of calculation".
He pointed out that a rounding-down approach operated by the IPS had also benefited Price.
Appeal judges will give their ruling later.