The Pope has failed to respond to a request for the Vatican to contribute towards the cost of excavating the remains of hundreds of children feared buried in the sewage system of a former Bon Secours mother and baby home.
Children's Minister Katherine Zappone said she had received no response since raising the issue with Pope Francis during his visit to Ireland in August.
The revelation came as the minister announced Government approval for a plan to exhume remains in the mass grave in Tuam, Co Galway.
It is part of a major forensic investigation expected to cost between €6m and €13m.
The Bon Secours Sisters has said it would donate €2.5m towards the project, an offer that has been accepted in principle by the Government.
However, campaigners said the offer was "too low".
Historian Catherine Corless, whose work helped expose the scandal, welcomed the minister's announcement, but said the order should pay at least half of the eventual cost.
"The order made money from those children. There is no doubt about that. And they neglected them severely," she said.
Anna Corrigan, of the Tuam Babies Family Group, said the Government had made "an exceptionally important decision", but added that there was "an onus on the order to put more money forward".
Ms Zappone said she still hoped to receive a response from the Pope and believed the Vatican would pay attention to the announcement.
The plan for Tuam involves a phased forensic investigation of the site, and the recovery of the remains "in so far as is possible". This will first involve test excavations to locate potential burials. Then there will be forensic analysis of any recovered remains and, where possible, their identification via DNA testing.
Arrangement will also be made for a respectful burial, memorialisation and the conservation of the site.
"Every effort will be made to locate and recover all juvenile remains from the site," said Ms Zappone. "What happened in Tuam was part of a pattern of injustice we cannot overcome unless we acknowledge it."
Unmarried pregnant women were sent to the home to give birth between 1925 and 1961.
They were separated from their child and required to do unpaid work in the home for a year. The children stayed until they were fostered, adopted or sent to industrial school.
But Ms Corless discovered 796 child deaths at the home were registered where no burial location was recorded. In March of last year, the Commission of Investigation into Mother and Baby Homes reported that significant quantities of human remains had been discovered at the site.
Ms Zappone could not say when the excavation work would begin as legislation needed to be passed first.
She did not rule out the possibility of similar exhumation projects at other mother and baby homes where there are known unmarked burial grounds. So far, however, Tuam is the only site where the commission has confirmed the presence of juvenile remains.
Ms Zappone said the Government's decision was informed by detailed technical advice on international best practice.
A report by the special rapporteur on child protection, Dr Geoffrey Shannon, had a significant bearing on the decision. He advised that a failure to provide relatives with credible information may fall foul of the State's obligations under the European Convention on Human Rights (ECHR).
He said family members might have the right to have their loved one's body returned to them and to know the fate of family members. Dr Shannon also concluded there appeared to be an inferred duty on the State to collect, as far as possible, the remains.
In his report, he noted that an expert group had found the Tuam site would test the boundaries of forensic investigation. He said that if the State failed to retrieve the remains, it would be unlikely to fall foul of its duties as long as all reasonable steps were taken to attempt retrieval.
In a statement, the Bon Secours Sisters said it acknowledged the pain experienced by those affected. It said it recognised the need for urgency in dealing with Tuam and had made "a substantial offer to help expedite matters".
The statement also claimed the revelations came "as a complete shock" and that the order had been unable to investigate what happened, as it no longer had access to the home's records.
The local historian who helped uncover the remains of babies dumped in the Tuam mother and baby home said she believes there are many mass graves to be uncovered around Ireland, but Tuam "is a little bit worse, the fact it was a sewage area".