Eir could be facing multi million euro fines for acting in an anti-competitive manner towards rivals that use its network.
The State’s largest telco has been accused of discriminating against other operators regarding fault repair times and presiding over unjustifiable delays in providing access to its wholesale network.
In five separate findings, regulator Comreg also claimed Eir had shown a lack of transparency in relation to how its services work.
The watchdog has given Eir four weeks to respond to the findings following which it will decide whether to initiate legal proceedings in the High Court, which could result in hefty fines for Eir.
In a statement, Eir said it had rectified the issues involved and would “vigorously defend” its position in relation to equal access and non-discrimination.
Access to Eir’s broadband and fixed-line network remains a contentious issue amid allegations the former semi-State favours its own retail arm over rivals that purchase access to the network.
The market incumbent has been in dispute with rival telcos - Sky, Vodafone, BT and Magnet - which use the network to deliver their own bundles.
Alto, the umbrella group that represents non-Eir firms, meanwhile, welcomed ComReg's decision to act to put an end to what it described as " abusive behaviour in the Irish telecom market".
"Eir must finally recognise that for an effectively competitive environment in Ireland, full compliance with regulation must be at the forefront of their agenda," Alto chairman Ronan Lupton said.
“ Not only are we clients of Eir, but we are serious competitors,” he said.
Mr Lupton also noted that ComReg’s opinions of non-compliance arise at a time when Eir was decrying the lack of access to other State infrastructures.
Eir recently accused the ESB of stonewalling requests for access to its infrastructure.
Under the EU’s shared utilities directive, infrastructural companies such as the ESB are obliged to open up their networks to other operators for strategic telecommunications projects.
The ESB, through its joint venture with Vodafone - Siro, is competing with Eir for the Government's national broadband contract.