Molly Martens Corbett and her father Thomas Martens have won their appeal against their convictions for the murder of Irishman Jason Corbett five years ago and will face a retrial.
The 39-year-old father-of-two from Limerick was found dead at his home in North Carolina in August 2015.
His wife Molly and her father, a former FBI agent, were convicted of second-degree murder following a trial in 2017.
Last January, lawyers for the pair told the North Carolina Court of Appeal that they did not get a fair trial and that the trial judge had excluded critical evidence in their case.
Prosecutors told their trial in 2017 that Thomas Martens and his daughter beat Mr Corbett to death with a brick and a baseball bat, administering at least 12 blows to his head and crushing his skull.
Thomas Martens claimed he acted in self-defence, testifying that Mr Corbett was choking his daughter, and he reacted to what he thought was some sort of disturbance.
The jury did not agree and both were sentenced to 20-25 years in prison.
In their appeal, lawyers for Molly and Thomas Martens argued that the judge in the case had excluded key evidence that would have corroborated their client's claims.
This included information which they said might have explained Thomas Martens' state of mind on the night of the murder.
They argued that statements Mr Corbett's children had given to social workers should have been allowed as evidence.
They also claimed the jury in the trial had disobeyed instructions from the judge not to have conversations among themselves about the case.
In its ruling, the North Carolina Court of Appeal said both defendants are entitled to a new trial.
The ruling stated: "Due to a number of prejudicial errors apparent within the record, we reverse the judgments entered upon Defendants' convictions for second-degree murder and remand for a new trial."
The court described its ruling as "deceptively simple", boiling down to whether the defendants lawfully used deadly force to defend themselves and each other during the tragic altercation with Mr Corbett.
"Having thoroughly reviewed the record and transcript, it is evident that this is the rare case in which certain evidentiary errors, alone and in the aggregate, were so prejudicial as to inhibit defendants' ability to present a full and meaningful defence," the court ruled.
The judges also found that "the trial court erred in instructing the jury on the aggressor doctrine as to Tom, given the absence of evidence to support such an instruction".
The ruling by the three judge panel was not unanimous, with one of the judges dissenting in part.
Prosecutors have said they anticipate that they will appeal the ruling.
Garry Frank, the District Attorney for Davidson County in North Carolina, told RTÉ News that he was very disappointed by the judgement.
He said the trial judge in the case is one of the best in the district and that he did not agree that he had made errors during the trial.
Prosecutors now have 35 days to appeal the ruling to the North Carolina Supreme Court and Mr Frank said he will work on that appeal with the Office of the Attorney General of North Carolina.
A spokesperson for Jason Corbett's family said: "We are aware of the decision of the North Carolina Court of Appeals in relation to the retrial of Molly and Thomas Martens.
"We will not be commenting on the decision at this time."