A Health Information and Quality Authority report says inspectors in Dublin North West were not assured that all children in foster care were safe.
It says there is no evidence that lessons had been learned from past experiences.
The report says that some children remained in placements even though allegations had been raised about their foster carers.
Minister for Children Frances Fitzgerald has described the report as unacceptable.
She said: "Clearly in 2010 it was recognised that this area had a series of problems, this is HIQA's third visit.
"It's good that there are some improvements. The underperformance is, however, unacceptable.
"I will be urgently meeting the HSE and Gordon Jeyes, Chief Executive Designate of the Child and Family Support Agency in order to review the actions under way to address the critical issues identified."
A second report into foster care in Limerick found the standard of social care was good, but not all safeguarding arrangements were robust.
It says a number of children were living with unapproved carers and it is not possible to ascertain if all staff were properly vetted, trained and qualified, as the records were not adequate.
The Limerick area has 276 children in foster services.
The report covers foster services in Dublin North West, which encompasses Clonsilla, Finglas, Blanchardstown, Glasnevin, Castleknock and Cabra.
The inspections were carried out there in October of last year and found 368 children in foster care services; 203 of whom were in general foster care, with 165 in foster care with a relative.
It found that the majority were in safe care.
However, it says Children First, which is the national guidance on the protection and welfare on children, was not being fully and consistently implemented in the area.
It states that 38 allegations and concerns about foster carers had been made in the last 12 months and a significant number had been confirmed.
Many of the children, it says, remained in their placement.
It also points out that some children were in placements that had not been approved by foster care committee and there were significant delays in investigating allegations.
It says not all of the social workers' personnel files that it reviewed contained the necessary Garda vetting and there is insufficient evidence to confirm that social workers had received training in Children First.
As of their last inspection in Dublin North West, it said the register for children in foster care was not up-to-date.
The report found 41% of those in care were placed in foster homes in different counties which made maintaining contact with their families difficult - this was due to lack of foster placements.
It said the Dublin North West area was insufficiently resourced to enable larger groups of siblings to be placed together and this resulted in siblings being separated.
It also said there is no overarching risk management process that tracked and addressed complaints and allegations against foster carers and no evidence that lessons had been learned from past experiences.
The report found that 116 of the children had been assigned a social worker three weeks before an inspection.
Health and safety checks were not carried out in every foster home. It also raised the issue about access to aftercare facilities.
A second report into foster services in Limerick, which has 276 children in foster care, found the standard of social care was good in the area.
However, there was an insufficient number of foster carers and there was lack of emergency placements for teenagers.
It also found that not all safeguarding arrangements were robust - the foster care committee was not informed of concerns and allegations raised about foster carers nor was it aware of the findings of foster care reviews.
In general it says foster carers were not updated on current child protection issues.
The report says a number of children were living with unapproved carers.
It also states that the HSE could not provide evidence of garda vetting from the personnel files of 15 social work staff.
The report noted that it is also difficult it says to ascertain the overall number and status of complaints made by children in order to audit them.
It says it is not possible to ascertain if all staff properly trained and qualified as records were not adequate.
HSE accepts findings
The Health Service Executive said it has considered the report and fully accepts the findings.
It said that in relation to service delivery and management structures, "much remains to be done and the pace of improvement is too slow".
The executive said it is committed to addressing the issues raised in the report.
Fianna Fáil's Robert Troy said the report was "shocking and extremely worrying," and called for a response from Ms Fitzgerald.
Elsewhere, the Special Rapporteur on Child Protection, Geoffrey Shannon, said we need to learn from past mistakes when it comes to the treatment of children in State care.
Speaking on RTÉ's Six-One News, Mr Shannon said child protection is not going to change overnight.
He said Minister Fitzgerald has been very proactive in bringing forward a number of initiatives such as the mandatory child protection guidelines and new children first legislation.
Mr Shannon described the report as troubling.