The apologies were issued at in-person meetings with patients and their families in recent weeks.
However, an official report on the issue at South Kerry Child and Adolescent Mental Health Services (CAMHS) will not be published until the new year.
The review was launched last April amid fears some patients of the service were prescribed dangerously excessive amounts of medication.
It has involved the examination of the files of more than 1,500 patients.
Coleman Legal, a law firm representing more than 60 affected patients, has raised concerns about the process, saying it emerged during the meetings with HSE staff that records relating to many patients had gone missing.
These included appointment records and other key information relevant to their care.
Health Minister Stephen Donnelly is to be briefed on the findings of the review once it is delivered.
In at least some cases, it was feared patients were prescribed inappropriate medication dosages and may have suffered significant health issues as a result.
The controversy is expected to lead to a raft of lawsuits.
Proceedings have already been issued by some of Coleman Legal’s clients.
The review is being led by Dr Seán Maskey, a consultant specialising in child and adolescent mental health at Maudsley Hospital in London.
The review team includes senior nurse managers, advanced nurse practitioners and administration staff, and is focusing on care given between July 2016 and April 2021.
While its work is substantially completed, a final report has yet to be provided to the HSE.
In a statement, the HSE said that where it was found a young person did not receive the standard of care they should have, meetings were arranged with them and, where appropriate, their families. “We are apologising to each individual at these meetings for any harm caused to them,” the statement said.
“All of the families and children have either now been met or have been offered the opportunity to meet for the purposes of open disclosure and to provide them with key information.”
The issue first came to light in the autumn of 2020 when a member of the South Kerry CAMHS team became concerned about some of the clinical care of patients they were treating.
A sample review involving 50 patients took place, and the findings were deemed significant enough for a large-scale review to be ordered.
The matter was also referred to the Medical Council.
Keith Rolls, a partner with Coleman Legal, said the firm had written to the HSE and Mr Donnelly to document concerns in relation to the review.
“A predominant concern is the extent to which medical records relating to the treatment afforded to our clients are missing,” he said. “The absence of complete records, for whatever reason, has simply added to the failures and the confusion and hurt for our clients and their families.
“These patients are extremely vulnerable individuals and have presented to the HSE team to receive the care they undoubtably need. Sadly, this has not happened to date.”
Asked about the absence of patient records, the HSE said: “In relation to questions about the review process and the contents of the final report, we have not yet received the final report from Dr Maskey and that we cannot speculate on its contents or undermine the independent process under way.”