Eircom has revealed that the personal details of 6,845 eMobile and Meteor customers and 686 Meteor staff were contained on three of its laptops that were stolen.
The bank account details of 550 customers may also be at risk.
Although Eircom has an encryption policy for its laptops, it is understood none of the three stolen were encrypted.
Two laptops containing customer details were stolen from Eircom's offices at Parkwest in Dublin over the Christmas period.
On them were the details for 6,441 current and previous eMobile business customers dating from August 2010 until December 2011.
In most cases the data at risk includes names, addresses and telephone numbers, however for 146 customers financial data including bank account details may be at risk.
There is also a risk for 404 Meteor post-pay customers, who applied online in the first half of last year.
For these people a range of documents they used to prove their identity in their applications - like passport or drivers licence details, photo IDs or utility bills - were included.
In some cases bank account, Laser or credit card details are also at risk.
In a separate incident in December, a laptop containing the names and addresses of 686 Meteor employees was stolen at an employee's home.
Gardaí were notified in early January and two investigations are under way.
None of the three laptops have yet been recovered.
So far there is no evidence that the data at risk has been used by a third party.
Eircom said its agents are in the process of phoning all 550 customers whose financial data is at risk.
The Irish Banking Federation said that it is facilitating engagement between Eircom and the banks whose customers are affected.
This will enable the banks to take the necessary steps to monitor the relevant customers' accounts and liaise directly with Eircom as appropriate.
The IBF said that this process has proved effective in such cases in the past.