For more than four years the Mulhall sisters have refused to say where they disposed of the head of their victim, Farah Noor.
Now Linda Mulhall has revealed the final piece of the mystery after she told fellow inmates that they emptied the fragments of Noor’s skull into bins in the Phoenix Park.
Gardai have been informed of the revelations and have launched an investigation.
Linda (31) made the grisly admission at the Dochas women’s prison in Dublin last month.
During her high profile trial, Mulhall maintained she couldn’t remember what she did with the head, which she had earlier buried, dug up and then hidden in bushes.
The Herald understands that Linda Mulhall took the decomposing head from undergrowth at Tymon Park North in Tallaght and brought it by bus to the Phoenix Park.
She told fellow inmates that, once in the park, she used a hammer to crush the skull into small pieces.
She then put some of the pieces in a plastic bag, before leaving the remainder in the original plastic bag it was first stashed in.
The two bags were then dumped in refuse bins.
The final resting place of Noor's head is now believed to in landfill.
A prison source said Mulhall made the admission shortly after her mother (53) pleaded guilty to a charge of covering up Noor's murder.
Linda Mulhall and her sister Charlotte are serving sentences for the manslaughter and murder of Noor respectively, at Richmond Cottages in Ballybough four years ago.
They killed and cut up Noor's body after a drink and drug fuelled row at their mother's home.
Noor had dated their mother for a period of time.
The prison source told the Herald: "Linda came out with the admission a few weeks ago. She said she couldn't remember at the time of the trial but that it had come back to her since.
"She went to Tallaght where she had put the head in bushes and found it.
"She then placed it in another bag and brought it back into the city. She got off and walked to the Phoenix Park.
"According to Linda, at this point she used a hammer to break up the skull, while it was still inside the bag.
"She then put half the crushed remains into a second bag, and dumped the two bags in two different bins," the source added.
The disclosure ends the four year old mystery surrounding the whereabouts of Noor's head.
The rest of his body parts were dumped in the Royal Canal after the murder, and later recovered by gardai.
However, the victim's penis has never been found.
The crime became one of the most gruesome murders in living memory, and earned Linda and Charlotte Mulhall the nickname the 'Scissor Sisters'.
Linda Mulhall has taken a case to the Supreme Court arguing that exceptional points of law should be considered in her case.
She is seeking to have the severity of her sentence reduced after previously losing an appeal against her manslaughter conviction.
Linda and Charlotte's mother, Karen Mulhall, will be sentenced on May 5 next, after she admitted to cleaning up the crime scene at her home after Noor was murdered there.