A mass grave containing the remains of babies has been found in the sewers of a former Catholic orphanage in western Ireland, according to investigators, confirming a local historian's suspicions of the unmarked burial of hundreds of children.

Excavations found "significant quantities of human remains" in an underground structure divided into 20 chambers at the site of the former Bon Secours Mother and Baby Home in Tuam, County Galway, a report from a government-appointed inquiry said on Friday.

The judge-led Mother and Baby Homes Commission said DNA analysis of selected remains confirmed the ages of the dead ranged from 35 weeks to three years old.

The investigators did not say how many babies' remains were recovered, or how many might still be buried in what are believed to be the home's sewage or waste water treatment system.

READ MORE: Can Ireland face up to its shocking past?

The announcement confirms decades of suspicions that the vast majority of children who died at the former home were buried in unmarked graves, a relatively common practice at such Catholic-run facilities amid high child mortality rates in early 20th-century Ireland.

In 2014, the government ordered an investigation after Tuam historian Catherine Corless said there was evidence of an unmarked graveyard at the facility.

Corless found death certificates for nearly 800 children who were residents at the Tuam home between 1925 and 1961, but a burial record for only one child.

"Everything pointed to this area being a mass grave," said Corless, who recalled how local boys playing in the field had reported seeing a pile of bones in a hidden underground chamber there in the mid-1970s.

Katherine Zappone, Ireland's minister for children and youth affairs, called the commission's findings "sad and disturbing", adding that the investigators would work with local authorities to decide about the next steps.

Ireland's once powerful Catholic Church has been rocked by a series of scandals over the abuse and neglect of children. The Archbishop of Tuam said in 2014 he was horrified and saddened by the historian's discovery.

Unmarried mothers and their children were seen as a stain on Ireland's image as a devout Catholic nation. They were also a problem for some of the fathers, particularly powerful figures such as priests and wealthy, married men.

Government records show that in the 1930s, 40s and 50s, the mortality rate for "illegitimate" children was often more than five times that of those born to married parents.

On average, more than one in four children born out of wedlock died.