The 400,000 users of Eir's webmail service are experiencing intermittent access to their accounts due to a Distributed Denial-of-Service (DDoS) attack on the company.
A spokesman confirmed the problem has been coming and going since Wednesday, when higher than normal levels of traffic were targeted at the company's servers.
Nobody has claimed responsibility for the attack, but it is understood the traffic appears to be coming from various places around the world.
The company is using special filters that detect when fake traffic is being directed at the webmail servers and redirects it.
The spokesman said that while the service was still operational, some users were experiencing difficulties in accessing their accounts at certain times.
In January a number of websites of the State bodies and companies, including Eir's mobile arm Meteor, were temporarily knocked offline by a large coordinated DDoS attack.