Forty-three children are believed to have been at risk after drain cleaner was mixed with water in a dental clinic.
Health chiefs said they were trying to contact the parents of the youngsters who were treated at the Health Service Executive (HSE) facility on Bindon Street in Ennis, Co Clare.
The incident is believed to have occurred on Wednesday and went undetected until Thursday.
The dental clinic, where five of its six surgeries were operating during the scare, has been shut as a result and is not expected to reopen until Monday or Tuesday.
The HSE said that e arly indications, yet to be confirmed, suggested that a drain cleaning product had become mixed with the water used during treatment.
The water should have been mixed with an approved dental product, it said.
HSE Mid West chief officer Bernard Gloster said: "This is so regrettable and unfortunate.
"I want to apologise for any concern or upset caused to the children and their families.
"We are taking all of the necessary steps to ensure they are supported and have adequate information. We will be conducting a full review to establish all of the facts.
"I am grateful to all of our management staff and clinicians who worked very hard late into last night to respond to this incident."
It is understood the cleaning product was hea vily diluted but inadvertently attached to the rinse aid used by children and patients while in the dentist's chair.
A dentist raised the alarm after noticing a child's reaction when told to rinse their mouth following treatment.
Two other children were taken to A&E after complaining of mild irritation to the mouth and feeling ill.
Other children visited GPs after concerns were raised.