Prosecutors claimed CCTV evidence backs their case that Damien McLaughlin transported a Toyota Camry vehicle across the border on the eve of the dissident republican attack on David Black.
Mr Black, 52, was gunned down on the M1 Motorway near Lurgan as he drove to work at HMP Maghaberry on 1 November.
McLaughlin, 36, of Kilmascally Road, Dungannon, Co Tyrone, denies a charge of preparation of a terrorist act.
He was refused compassionate bail to attend his child's christening service on Saturday.
The court heard how the car used by the killers was bought in Dublin for £600 through an online advert last October. False details were supplied by the purchaser, who is not alleged to be McLaughlin, according to the prosecution.
The car was said to have been moved to Carrigallen, Co Leitrim where it remained for nearly three weeks.
A prosecution barrister claimed CCTV footage shows McLaughlin in the village on 31 October.
It was alleged that he obtained a car battery used to start the vehicle.
Later that evening it was known to have crossed the border into Northern Ireland, leaving the M1 near Lurgan, the court heard.
Mr Black was shot dead the next morning after leaving his Cookstown home to head for the high security prison. The Toyota Camry was later found burnt out.
Opposing McLaughlin's bid for temporary release, the prosecution lawyer outlined police fears that he may flee.
"They believe he has connections to dissident IRA groups," she said.
The barrister also stressed that McLaughlin is not suspected of just having a periphery involvement.
She added: "The police case, let's be firm about it, is this applicant is part of a highly organised criminal gang intent on the assassination of a member of the Prison Service, and that he formed an integral part of that gang."
Defence counsel Mark Mulholland QC argued that his client must be presumed innocent.
He claimed the case against McLaughlin was "sparse", limited only to him allegedly being in the car the night before the murder.
"There is no suggestion of any active role in it," Mr Mulholland insisted.
He disclosed that a priest was prepared to chaperone the accused at the christening in Ardboe.
Relatives were also prepared to lodge £13,000 in cash sureties and property deeds to secure bail.
But refusing the application, Mr Justice Treacy pointed out that McLaughlin's wife would have been heavily pregnant at the time he was alleged to have been involved in the terrorist plot.
The judge added: "It is untenable to expect or reasonably contemplate in those circumstances this court or any court would release this applicant on bail given the grave risks that would give rise to."