The Health Information and Quality Authority (HIQA) has found standards on safeguarding and reviews of foster carers is "majorly non-compliant" in three areas.
It also found that garda vetting had not been updated and elements of overcrowding in some foster care households.
It has published three inspection reports on foster care services operated by the Child and Family Agency (Tusla).
They relates to services in the Dublin South Central, Cavan/Monaghan and Dublin North City.
HIQA found while the standard on assessment and approval of non-relative carers was found to be compliant or substantially compliant in all three service areas, the standards on safeguarding and reviews of foster carers was judged to be "majorly non-compliant".
In all three inspections, inspectors found that there were not always appropriate safeguarding arrangements in place, child protection concerns were not always managed in line with the Children First policy, and routine reviews of foster carers were not taking place.
HIQA says: "This meant that the service could not ensure that foster carers had the continuing capacity to provide safe care.
"Garda Síochána vetting had not been updated and foster carers' performance, training needs and support requirements had also not been reviewed."
Of the eight standards assessed, seven were found to be non-compliant, two of which were judged to be majorly non-compliant.
These included safeguarding and child protection and reviews of foster carers.
HIQA says: "Good practice was found in relation to assessments of both general and relative foster carers, though there were some delays in completing assessments of relative carers.
"The quality of supervision, when it did occur, was good.
"However, there were insufficient foster carers in the area to meet the demands of the service.
"At times this resulted in more children being placed in a foster care household than is recommended.
"The foster care committee in the area was not fully compliant with national policy."
"While there were a number of safeguarding measures in place, not all foster carers or members of foster care households who were over 16 years were garda vetted."
Dublin South Central
One standard was compliant, while seven were found to be non-compliant.
Six standards were found to be in major non-compliance.
HIQA says: "Similar to an inspection of this area in November 2016, the management of the service was crisis led rather than delivered in a planned manner.
"Inspectors found that improvements had been achieved in relation to garda vetting of all staff.
"However, the area continued to be challenged by staff vacancies which impacted on the completion of assessments, support and supervision of foster carers and the completion of up-to-date reviews of foster carers."
HIQA also identified a number of serious risks - including long delays in the commencement and completion of relative foster carer assessments and in achieving a decision from the foster care committee.
It also says not all children received a timely and appropriate response when a child protection concern was made.
Other safeguarding components, including supervision and support of foster carers, the use of safety plans, updated garda vetting of foster carers and garda vetting of members of foster care households who were over 16 years, and training were inadequate.
Dublin North City
Of the eight standards assessed here, two standards were substantially compliant, and six standards were found to be non-compliant.
Five standards were judged to be in major non-compliance.
It says: "Good quality assessments of general foster carers were being carried out, though not always within the 16-week time frame set out in the standards.
"The service had significant challenges in completing assessments of relative carers and there was drift and delay with some of these assessments.
"Assessments carried out by Tusla were of good quality, but there were gaps in Tusla's oversight of those undertaken by private agencies."
It also says appropriate safeguarding arrangements were not in place for all foster carers here.
"Not all members of foster care households who were over 16 years of age were garda vetted and foster carers did not always have regular home visits with their link worker.
"In addition, 30 foster carers in the area had not been allocated a link worker and more needed to be done to ensure that these carers received adequate support and supervision."
In conclusion, HIQA says all of the service areas have provided action plan responses to address the non-compliances identified on inspection, which contain timelines for implementing these actions.
Commenting on the reports, Tusla chief operations officer Jim Gibson says: "HIQA inspections are an important measurement and oversight tool for us in Tusla and allow us to ensure that our services operate at the highest possible standard.
"These reports show evidence of positive practice and improvements in services.
"The reports also identify areas for improvement which we are targeting through comprehensive action plans which were agreed with HIQA."
“This work takes time and we are committed to implementing the required actions as promptly as possible to improve the service for children, families and our foster carer community."
It has also outlined measures in place to improve the services and standards.