Linda Mulhall has lost a bid to bring a Supreme Court challenge to her 15-year sentence for killing her mother's boyfriend.
The headless torso of Farah Swali Noor was found in the Royal Canal in Dublin in 2005.
He had been killed and cut up by Linda and her sister Charlotte.
Her younger sister Charlotte is serving life for the brutal murder of Farah Swali Noor, described by sentencing judge Mr Justice Paul Carney as "grotesque".
Linda Mulhall has already lost an appeal against the severity of her 15-year sentence for manslaughter but in this action, her lawyers argued that that decision should be reviewed by the Supreme Court.
It was argued her case raised four points of law of exceptional public importance.
She was not in court for today's ruling dismissing her case on all grounds, which effectively ends her bid for early freedom.