The Adoption Rights Alliance (ARA) say 126 illegal adoptions discovered by Tusla in records from St Patrick's Guild in 2015 is 'a fraction' of the total number.
The organisation says its immediate concern is for the people affected by these illegal adoptions.
"The effects of illegal adoption on both the adopted person and the natural mother cannot be understated", it says.
"We urge the Department of Children and Youth Affairs to put in place the necessary supports for both the adult adopted people and the natural mothers involved.
It's also offering peer support to people affected, on its private Facebook group.
Yesterday it emerged dozens of people placed for adoption in St Patrick's Guild may not know they are adopted.
The child and family agency Tusla identified 126 cases where births were incorrectly registered by the former agency between 1946 and 1969.
They are cases where the adoptive parents of the child were recorded as the birth parents, without an adoption order.
Seventy-nine of the people affected may have no idea they were adopted and "may be entirely unaware of the true circumstances of their birth".
An independent review has been ordered into the incorrect registration at St Patrick’s Guild.