Graham Dwyer has won his legal action against the State and Garda Commissioner over the use of mobile phone data in his murder trial.
The former Foxrock architect, who was sentenced to life in prison for the murder of Elaine O'Hara in 2015, claimed that the use of information from his work phone breached his right to privacy.
The data was a central part of the prosecution case because it placed him in a specific place at a particular time.
The High Court judge ruled the 2011 law which allowed access to Dwyer's phone and the subsequent retention of data breached EU law and the European Convention on Human Rights.
This means Dwyer has won this stage of the legal battle and it will now progress to the Court of Appeal where the use of phone data will be argued further.
Dwyer, who is serving a life sentence in the Midlands Prison for murdering the childcare worker in the Dublin mountains, wasn’t present in court for the 100 page judgement.
The Justice Minister described it as a very detailed and lengthy judgement, which will have to be considered very carefully to identify exactly what actions are necessary.