A ruling by the Communications Regulator, which found that Vodafone and O2 dominate the Irish mobile phone market, has been annulled.
The decision by the Electronic Communications Appeal Panel to overrule COMREG means that a full review of the market will now be carried out.
It comes after a number of changes to the mobile phone market in recent months with the launch of new operator "3", and the re-entry of eircom.