At least 25 cases of mystery hepatitis in children have been identified in Ireland since March, according to infectious diseases experts.
Two children needed a liver transplant after becoming infected, while another child died, the Health Protection Surveillance Centre (HPSC) said.
One possible case of severe, acute hepatitis of unknown cause has also been identified, and a small number of potential further cases are currently under investigation. All but one child required hospital care.
In common with experts around the world, the HPSC is attempting to learn more about this new disease, which is of unknown origin.
While none of the affected children who were PCR-tested for Covid-19 were positive for the virus, 70 per cent were positive for Sars-CoV-2 antibodies, which is indicative of current or past infection.
More than half (52 per cent) tested positive for adenovirus, which is suspected by many scientists as a possible cause of the disease.
Human herpes virus was identified in three of the six children tested, and small numbers tested positive for other harmful bugs.
Children affected range in age up to 12 years; the median age is four. The largest number of cases, 14, has occurred among one- to four-year-olds.
Two of the children were vaccinated, 18 were not and the status of the remaining five is unknown.
The main symptoms identified were malaise or tiredness, pale stool, jaundice, abdominal pain and vomiting.
The condition was first identified in the UK last April, after scientists there had excluded all known types of hepatitis as the cause of the liver inflammation they were seeing among some young patients.
Last week, the European Centre for Disease Prevention and Control (ECDC) reported a total of 508 cases across 21 countries. Only the UK, Italy and Spain have more cases than Ireland. Case numbers show a decline in recent weeks.
The ECDC says a possible association with current adenovirus infection has been identified, but other hypotheses and possible co-factors are under investigation. Most cases continue to be reported as sporadic, unrelated cases.