So called "tandem burials" happened at a limited number of hospitals in the Republic of Ireland up to the 1980s.
It is understood this happened in what the authorities called "exceptional circumstances".
This might be if a newborn baby died and options for burial in a hospital or religious plot were not selected.
The remains would then be placed in the coffin of an adult who had died.
Details of such burials were set out in documents obtained by RTÉ News under the Freedom of Information Act.
In a letter to the former secretary general of the Department of Health, Dr Ambrose McLoughlin in August, Irish Health Service Executive director general Tony O'Brien  said it was not certain that the families of both of the deceased would always have been informed of the practice.
In the letter, Mr O'Brien said: "Our understanding is that the remains would have been placed with adult remains and that, ideally, the awareness and understanding of both families involved would have been sought, though this is not guaranteed."
It is understood that the practice may have occurred in a number of cases to ensure that a baby was buried in consecrated ground.
At a time in Ireland, the Catholic Church's tradition stipulated that if a baby was born and died before baptism, they could not be buried on sacred ground in a cemetery.
The souls of such babies would go to Limbo - an in-between place where they could neither see God nor be reunited with their parents.
In 2007, Pope Benedict XVI revised the concept of Limbo.
In Ireland, dead babies have been buried in the coffins of adults to whom they were not related, according to Irish broadcaster, RTÉ.