One In Four has called on those aware of the past abuse of others to speak up.
The organisation's latest annual review shows it supported hundreds of men and women in "an under-resourced child protection system" and a society in which "the bystanders ... chose not to act".
The annual review shows that One In Four last year supported 673 men and women who had experienced sexual abuse in childhood, as well as 65 men who had caused sexual harm to children. Of the abusers who were treated in the programme, 48% had committed an online offence and there was also an increase in younger offenders, with 7% aged 18 to 25.
It also met 56 people for a first assessment meeting and found that 43% of clients were abused in their own family, with 37% of the clients being men.
The huge number of survivors assisted, including through the delivery of 2,398 individual and group psychotherapy sessions to 125 clients, was despite having to close the waiting list for psychotherapy for four months in 2021 as the waiting period went beyond one year. One In Four created the new position of waiting lists manager - supported by Tusla - to support the crisis counselling to people on the list.
However, the organisation said several issues persisted, not least the under-reporting of child sexual abuse, and the adverse journey many face through the criminal justice system when cases are investigated.
CEO Maeve Lewis said questions needed to be asked as to why the process was so difficult, including what she described as "structural barriers", particularly when survivors were already dealing with the trauma of what happened to them earlier in life.
"That is why so many sex offenders can operate with impunity. We have an under-resourced child protection system which cannot hope to properly investigate every notification of alleged sexual abuse, particularly when the victim is now an adult and the abuse is historic."
Ms Lewis said An Garda Siochána had made "huge strides" in recent years but that issues still remained, including the "great distress" of criminal trials.
"In 2021 we supported 53 clients through a trial," she said. "Despite very positive changes and a more trauma-informed approach introduced in the past number of years, people still describe this experience as re-traumatising, humiliating and de-humanising.
"Why is the onus to disclose always on the person who has been abused? Where are all the people, the bystanders, who must have been aware that something was wrong and who chose not to act? Despite all the progress of the past 20 years, Ireland is still a place where one in four children experience sexual harm.
"We believe that these latest shocking disclosures must become an opportunity to drive systemic change and to tackle the root causes of why sexual abuse is so pervasive."