Eir have said that the security breach which has affected around 37,000 Eir customers is the result of the theft of one laptop.
The laptop, which was stolen offsite, had been encrypted as well as password protected in line with company policy.
However, due to a faulty security update the previous working day, the laptop had been decrypted.
The faulty update affected a number of the company laptops which have since been fixed.
Customers whose data may be at risk are being contacted by Eir and customers who need further information can find it here.
Earlier: 37,000 Eir customers hit by data breach after staff laptop stolen
Around 37,000 Eir customers have been hit by a security breach after a staff laptop was stolen.
The device includes names, email address, phone numbers and account numbers - but the company insists no financial data was exposed.
The incident has been reported to Gardaí and the Data Protection Commissioner.
Tech expert Jess Kelly says Eir is now in the process of contacting all affected customers.
"We don't know the extent of the damage that could be done by something like this
"At the moment, it doesn't look like it is a particularly malicious attack - it's just an inconvenience.
"So, I don't think that passwords need to be changed or anything like that."