The Revenue Commissioners has asked a Cork company operating a Local Property Tax helpline to review its procedures relating to the hiring of staff.
Credit card details of 11 callers are reported to have been improperly obtained by an Abtran call centre employee.
A number of unsuccessful attempts were made to use the credit card details to make purchases online, but no one suffered loss.
An Abtran employee has been suspended and gardaí have launched an investigation.
The matter has also been reported to the office of the Data Protection Commissioner.
Speaking on RTÉ's Morning Ireland, Revenue Chairperson Josephine Feehily said she was satisfied that the incident was limited to one member of staff at Abtran.
She said it was important to note that no customer incurred any loss and that the incident was dealt with speedily.
Ms Feehily said that the public should be reassured that the service operated for Revenue was professional and competent and was constantly overseen by officials from her own office.
All of the safeguards that Revenue insists upon were in place at Abtran, she added.
"We take data security extremely seriously. Data is the lifeblood of tax administration."
She said the company involved satisfied Revenue they had the highest standards in place.
However, she said it was hard to legislate for an incident involving one person.
"Over the weekend we have been working very closely, in hourly contact to make sure that we knew exactly what was going on, that we were taking all the steps necessary to mitigate this risk," she added.
Revenue established a telephone helpline when it announced details of how it planned to administer the Local Property Tax in March.
The contract to operate the helpline was won by Abtran, which provided 250 employees.
Gardaí have alerted the credit card companies involved, and they and the Revenue are attempting to make contact with the cardholders involved.
Pat Ryan, Client Services Director at Abtran has apologised for any inconvenience caused to customers.
Speaking to RTÉ News Mr Ryan said that he believes that the company's own safety and security measures would have identified the problem had it not been brought to its attention by a customer.
Mr Ryan said that the company responded immediately to the issue, and it will now review all its processes.
"When we did hear about this, we acted very quickly, we acted promptly, and when we acted we didn't think about Abtran, we thought about the customer”.
The Revenue Commissioners say more than 400 people have contacted their staff through a dedicated phone line set up to deal with people who have concerns over the incident.
Revenue officials are staffing the phone line, which is 1890 226336.
The phone line began operating at 9am.
A spokeswoman for the Revenue Commissioners said 420 people had made contact by midday.