Eir is warning customers that its modems have been compromised in a security breach.
It is understood at least 2,000 have been breached by a computer virus.
Customers are being advised to reset their devices after a security vulnerability identified on a "limited number" of Zyxel D100 and Zyxel P-660HN-T1A broadband modems.
"We strongly recommend that customers with these modems change both their modem administration password as well as their WiFi password as soon as possible," the company stated.
The company is contacting those affected by letter and email to inform them of the situation.
Full details of eir’s advice to its customers can be found here.