RTÉ Investigates found a case where despite the existence of "credible" allegations of sexual abuse, Tusla fought to keep a child in a foster placement where the abuse was alleged to have occurred and without removing the risk itself.
It comes as the Irish Society for the Prevention of Cruelty to Children called for immediate action to improve child protection, after a report called for a complete cultural change in the system.
In 2011 a young boy living in voluntary foster care, named 'Mark', told his birth mother he had been inappropriately touched by another male foster child twice his age. This child was also living with the same foster family.
The mother of 'Mark', 'Susan' said that she was distressed when told about the alleged abuse by her son, and disclosed this to the foster mother.
She said: "Obviously I was shocked ... he was quite distressed about it so when the foster mother came to pick him up I spoke to her about what my son had disclosed to me."
'Susan' discovered 'Mark' had made a similar allegation to his foster parent five weeks earlier but it had not been reported to the Health Service Executive.
She asked to have 'Mark' moved from the foster placement while an investigation took place, but this did not happen.
Instead a safety plan was put in place by the HSE, which consisted of little more than an instruction to the foster parents that the two boys, who had been sharing a bedroom, "should have separate bedrooms and that the [foster] carers should supervise" them closely.
Two months later, the HSE referred 'Mark' for assessment by child sexual abuse experts.
The experts concluded that 'Mark' gave "a credible account of experiencing inappropriate sexual behaviour by his foster sibling".
But while 'Mark' was assessed, it appears from case notes that his foster sibling was not.
Kieran McGrath, a child welfare consultant, told RTÉ Investigates that a proper evaluation of the older child was missing.
He said: "What was missing was a proper evaluation of the older young person and what his potential was to re-abuse.
"A much more adequate response would have been to take those steps - to assess the treatment needs of the victim child and also to look at a proper assessment of the older young person in terms of his needs and in particular the risk he might pose to someone else."
Over the following months, 'Mark' displayed sexualised behaviour and 'Susan' repeatedly reported this to the social work department.
'Susan' believed her concerns were not being taken seriously by social workers so she reported the matter to gardaí asking them to investigate.
Fact cases notes seen by RTÉ Investigates shows that gardaí were "...quite irate..." at the management of 'Mark's' case by the HSE social work department.
The notes stated that gardaí: "Would have a different view of child sexual abuse..."
The HSE decided to leave 'Mark' in the foster home despite his birth mother's concerns.
Almost three years later, 'Mark' made a second allegation, this time to a Tusla staff member that he was being punched by the same older foster child and by an adult son of the foster family.
The foster parents were again advised by the social work department to "increase supervision" of the boys and 'Susan' was informed about the allegation.
'Susan' said: "I was shocked and upset. I left the building and I just cried. I felt so guilty because I had asked for the social work department's help and instead of helping, my son was not protected in my view."
In late 2015 Tusla applied to the courts for a full care order for 'Mark'.
As part of the court process 'Susan' was required to produce two psychiatric assessment reports on the state of her mental health, to which she agreed.
But what 'Susan' did not understand was that she would be personally billed by her doctor for the reports, €500 for the first report and €300 for the second one.
On one occasion she was forced to take out a loan from her credit union to cover the cost.
Mr McGrath said: "This is a matter initiated by the State, it's meeting the State's needs using an employee of the State and then somehow a private bill is sent to someone who's generally speaking in no position to pay it so that, I have never come across a circumstance like that before, it's to my mind completely strange and inappropriate."
In an initial statement to RTÉ, Tusla said it "does not require individuals to undertake psychiatric assessments at their own expense."
However when asked by RTÉ Investigates why 'Susan' ended up paying her doctor's bills, a spokesperson for Tusla said they "cannot comment on individual cases."
Last year the court ruled that 'Mark' will stay in care until the age of 18.
He continues to live with the same foster family to the present day, along with the older child alleged to have previously abused him.
In its statement, Tusla said it "carries out complete assessments of all allegations it receives of abuse of children…" and all care planning "…is done on the basis of an individual child's needs and best interests, taking his/her views into account…".
'Mark' and 'Susan' are not the actual names of the foster child and his mother