The personal details of 37,000 customers have been stolen from Irish telecom company Eir.
In a statement, the company said the details were on a laptop which was stolen offsite.
The device held data including names, email addresses, phone numbers and account numbers - but the company insists no financial data was put at risk.
It said company policy dictates that all laptops should be encrypted as well as password protected, however the stolen laptop “had been decrypted by a faulty security update the previous working day.”
Tech expert Jess Kelly said Eir is now in the process of contacting all affected customers.
“We don’t know the extent of the damage that could be done from something like this,” she said.
“At the moment it does not look like it is a particularly malicious attack – it is just an inconvenience.
“So, I don’t think passwords need to be changed or anything like that.”
The company said it “promptly” reported the incident to the Gardaí and the Data Protection Commissioner.
It said there is “no evidence at this time that the data at risk has been used by a third party.”
It said it has now started a “programme to contact those customers whose data may be at risk.” Customers can find further information about the breach here.