The HSE said the children were diagnosed with Verotoxigenic Escherichia coli (VTEC), last week at the creche, which has been identified as Dun Beag Creche, in Dunmore.
That is a serious form of gastroenteritis that can cause diarrhoea which may be bloody, vomiting, nausea, and high temperature. The majority of cases get better with no treatment and without hospitalisation.
However, the most serious complication is haemolytic uraemic syndrome which occurs in up to 10% of VTEC cases and can lead to anaemia and kidney failure and requires intensive medical treatment.
A HSE spokesman said an outbreak control team is investigating with all children and staff submitting stool samples for laboratory testing. That has identified four further cases.
The part of the crèche where the affected children were was closed initially.
The HSE said children and staff will not be able to return to the creche until they have had two negative stool samples.
“Children or staff who have VTEC bacteria in their stool will not be allowed back into creches or other childminding centres until they are free of infection because of the risk of infecting others.”