"This was a crime against these people and their mothers. Mothers who were told that their babies had died," Barnardo head Fergus Finlay told RTÉ's radio show Morning Ireland.
On Tuesday, Irish Minister for Children and Youth Affairs Katherine Zappone revealed that Irish child and family agency Tusla had 126 confirmed cases of incorrectly registered births, Irish Central reported.
"I suspect every single adoption agency in the country is involved, that's 150,000 babies, it would be amazing if at least ten percent of them were not illegal," Finlay asserted.
"The bottom line is these people need their identity. They have a right to their background, who they are, where they came from. That trumps everything else," Finlay added, observing that a thorough investigation must be undertaken. "People have an absolute right to know."
Records from other Irish adoption agencies will be also be examined to determine whether illegal adoption practices occurred.
"I feel the likelihood is [that] other agencies were involved in similar practices, but I await the evidence before any conclusive comment can be made," Irish Minister for Justice Charlie Flanagan stated.
"I would find extraordinary if only one agency was engaged in these practices," Flanagan said.
The Adoption Rights Alliance (ARA) in Ireland has long urged for an investigation into illegal adoption practices in the country.
"We have repeatedly raised this issue for many years," Claire McGettrick of the ARA told Morning Ireland.
"Finally there is movement. We accept the mammoth exercise this is in the long term, we cannot leave anybody behind in this process. Every single file must be retrieved.
We have anecdotal evidence of ledgers in attics and of files being burned," McGettrick said, adding that "something needs to be done — sooner rather than later — to ensure every single record is centralized."
Last week, Irish Prime Minister Leo Varadkar described the discovery of the 126 confirmed cases as "another dark chapter in our history," adding that there may be "hundreds of thousands" of illegal adoptions awaiting discovery.