Tusla has denied this, saying it cannot compel people to make a disclosure
A sexual abuse charity says it is worried sex offenders are continuing to pose a risk to children, despite concerns being raised about them.
One in Four says it notified the child and family agency Tusla of 91 child abuse allegations last year and 12 victims agreed to make full statements.
It claims eight of these allegations were either not investigated or were deemed "unfounded".
Three investigations are ongoing and only one case came back as "founded".
It says in 79 cases, clients choose not to meet a social worker and it is very difficult to investigate without a full statement.
But it says only one sex offender is now being monitored.
One in Four executive director Maeve Lewis says this is very worrying.
"We are extremely worried that dangerous sex offenders may be continuing to abuse children even though we have brought them to the attention of Tusla.
"Our clients are adults who were sexually abused as children, but we know that sex offenders generally continue to abuse until they are caught.
"The father who abused his children may now be abusing his grandchildren: the teacher who abused one generation may now be abusing the next.
"Tusla has made strides in putting in place retrospective teams across the country, but our figures speak for themselves.
"From all these very substantial allegations, only one offender is now being monitored. We believe that Tusla child protection teams need much greater resourcing to deal with the volume of notifications."
In response Tusla says while it welcomes recognition of their positive joint working relationships in protecting children, it has denied the claim the agency did not investigate eight of the allegations brought to it.
The agency says it received 47,399 referrals to child protection and welfare services in 2016.
It says: "Tusla has a legal responsibility to assess the likelihood of any current or potential future risk to children.
"Where an allegation is made and a child is not deemed to be at immediate risk Tusla assesses the allegation to determine if they can be substantiated (founded or unfounded) and to ensure that we work with the child and family to protect the child from future harm.
"There are many cases where an allegation cannot be substantiated but Tusla remains involved to ensure a child is protected.
"Further, a finding of ‘founded’ is not the only basis for Tusla to provide a service to a child and family.
"The same set of obligations fall upon Tusla with regard to conducting assessments of retrospective allegations of child sexual abuse."
Jim Gibson, chief operations officer, at Tusla added: "Not all persons who have allegations made against them have criminal convictions for sex offences.
"We acknowledge that One in Four made a significant number of notifications in 2016, and a small number of clients made full statements to Tusla however, we refute the assertion that we did not investigate eight allegations.
"Where a person makes and allegation and decides not to engage with social work staff regarding the assessment, Tusla staff proceed on the basis of the information available and follow through as appropriate. We cannot, however, compel people to make a disclosure against their wishes."