Colm Murphy, 51, was sentenced to 14 years in jail in 2002 for aiding in the 1998 IRA bomb blast which killed 29 people and a pair of unborn twins.
A three-judge panel in Dublin's Criminal Court of Appeal granted a retrial on grounds that two detectives may have given perjured testimony in his original trial, the BBC reported.
In his original trial before a Special Criminal Court there was a question about alteration of interview notes and police testimony about them, the appeals judges said. They also said the trial court brought in Murphy's past criminal history, depriving him of the presumption of innocence, IrelandOn-line reported.
Murphy was originally convicted of lending two mobile phones to the gang who planted the bomb, knowing they would be used for moving bombs.
Michael Gallagher, whose son Aiden died in the blast, told the BBC he was in "total shock" at the decision.