Health watchdogs have found more than 100 foster carers have not been put through the required repeat vetting.
The Health Information and Quality Authority (Hiqa) said its review of childcare in Dublin South Central found significant protection risks.
It said Garda vetting did not always take place in line with the national standards and the policy of Tusla - the Child and Family Agency.
There are 351 children in foster care in an area covering parts of south and west Dublin including the South Inner City, Rialto, Inchicore and Ballyfermot, Clondalkin, Rowlagh, Palmerstown and Lucan.
Hiqa inspectors said records showed six foster carers whose vetting was outstanding.
There were another 107 carers who had not been put through repeat vetting within the three years deadline, as required under regulations.
Hiqa said Garda vetting was not always obtained for other adults living with, or who had unsupervised access to, children.
The inspection last November and December, which recorded 154 foster carers and 119 relative carers in 273 households, was told that there were 27 child protection and welfare complaints about children in Tusla's foster care in the area in the previous 12 months.
Some related to the foster parents but when files were checked three other concerns were found.
The files of two Tusla staff showed no evidence of them having gone through Garda vetting, the report said.
Hiqa said the management of concerns was inadequate and a significant risk to children.
There were also 17 incidents of children going missing from care in the Dublin South Central area in the previous 12 months.
The review found long delays in assessing children for fostering, a place in a care home, adoption, or an alternative such as moving them in with a relative.
It said some children had been waiting since 2012 for a decision.
In cases relating to relative foster carers, Hiqa found some children were in placements for up to six years before a comprehensive assessment had been completed.
The inspectors also said they found a small number of unapproved relative carers who had not appropriately engaged in the assessment process and where potential or known risks existed.
"Allowing a child to be cared for by an un-assessed and unapproved relative carer for several years posed a significant safeguarding risk," it said.
Inspectors also raised concerns that Tusla was carrying out repeat assessments of carers who had already been deemed unsuitable.
And they questioned why assessment periods for foster care applicants were being extended when risks had been identified.
"Records did not reflect valid reasons for delays or detail efforts to complete assessments," the report said.
Hiqa said matching children to foster carers was not always possible and 98 children were put in placements outside the area.
It also raised concerns about the lack of culturally appropriate placements.
Hiqa said social workers had heavy workloads and the service is under-resourced but on a more positive side inspectors found they promoted and respected children's rights and children were supported to maintain good relationships with their families.
Communication with children and families was described as respectful while the vast majority of children were said to have warm relationships with their foster families.
Hiqa also reviewed Care Visions, a for-profit agency providing foster care since 2013 and currently with 27 foster care households around the country and placements for 26 children.
It found significant risks on child protection and that the recording, reporting and oversight of incidents and significant events was inadequate, inspectors said.
"The monitoring and oversight of child protection and welfare concerns and allegations was not sufficient and there were significant gaps in the management and recording of child protection concerns," the report said.
Hiqa said Care Visions reported no allegations against foster carers or children in foster care prior to the inspection.
But inspectors found six separate child protection concerns related to allegations, for example, against foster carers, children in foster care and other adults and none of them were recorded in the child protection log.
Other records showed children had gone missing on five occasions in the 12 months prior to last November's inspection.
Tusla's chief operations officer Jim Gibson said: " Tusla remains committed to improving the care and safeguarding of children in foster care in Dublin South Central and all children in our care.
"In conjunction with Hiqa, we have created and begun to implement an action plan in Dublin South Central to address those areas identified as requiring improvement and have put in place a governance and oversight group to track progress on the action plan."
ISPCC chief executive Grainia Long said the review of Tusla services was disappointing.
"All children require robust systems for their protection, systems that are resourced, and that are followed. Foster care is now the responsibility of Tusla, the child and family agency. There is a need for a significant focus in this area, as a matter of urgency," she said.
"There must be comprehensive assessments and ongoing monitoring of foster placements."
Cathy Jamieson, Care Visions managing director, said the agency "recognises its responsibility to uphold child safeguarding practices to the highest standards possible and regrets that it did not fully adhere to the national standards in a number of categories".
She added: "In response, Care Visions has taken immediate steps to enhance the resources of the organisation and follow through on each of the requirements as highlighted by Hiqa in its report."