Lawyers for convicted murderer Graham Dwyer have begun a legal action against the garda commissioner and the State over data from mobile phones.
Data from mobile phone companies played a central part in Dwyer's trial three years ago for the murder of Elaine O'Hara in 2012.
His lawyers argue the legislation under which the data was kept breached his rights under the Constitution, the European Convention on Human Rights and the Charter of Fundamental Rights of the European Union.
Dwyer's Senior Counsel, Remy Farrell, said there was unlikely to be much factual controversy between the two sides. He said it was going to be quite a limited challenge of a legal nature.
Both sides intended to call evidence, he said, but he expected most of the evidence to come from the defence.
Mr Farrell has been outlining the relevant domestic and EU legislation to Mr Justice Tony O'Connor.
Dwyer's father Sean is in court for the hearing, as well as gardaí who investigated the murder.
Mobile phone metadata was an important part of the prosecution case in the trial.
This information, from mobile phone service providers, allowed the prosecution to show Dwyer's mobile phone was in certain places at certain times and allowed them to show where he was when calls and texts were made and received.
Gardaí were able to obtain this information under 2011 legislation brought in following a European directive.
The legislation obliges service providers to hold on to the data for two years.
But the directive was declared invalid by the Court of Justice of the European Union in 2014 because it interfered with fundamental rights including the right to privacy.
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A subsequent ruling by the court in 2016 found the way the act requires such data to be automatically retained is incompatible EU law.
The admissibility of the data was challenged by Dwyer's lawyers at his trial in 2015 but allowed in by the judge.
Mr Farrell said there were competing interests between the usefulness of the data for law enforcement officers investigating offences and data protection and privacy rights.
He said they had a fundamental difficulty with the indiscriminate regime under the act allowing everyone's data to be retained for two years. He added that they had complaints about the access to this data, particularly the lack of independent oversight in relation to accessing the data.
Mr Farrell told the court that Ireland had opted for the maximum possible period allowed under the directive to retain data and had gone for the most intrusive approach possible. It had also gone for a very broad definition of the types of offences covered by investigating authorities seeking the data, he said.
The court heard that Chief Superintendent Peter Kirwan applied for metadata relating to Dwyer's phone on 3 October 2013.
Mr Farrell said the application was made for data going as far back as they could. Two weeks later, Dwyer was arrested for Ms O'Hara's murder.
Mr Farrell said a significant part of Dwyer's questioning and his trial focused on Dwyer's phone and a substantial element of the case was trying to link him to two other phones found in Vartry Reservoir.
The gardaí and prosecution used the phone metadata in the clearest possible way he said, to establish Dwyer's movements and routine and link him to the phones.
Dwyer is seeking declarations that the provisions of the 2011 legislation are incompatible with EU law and in breach of his rights under the Constitution, European Convention on Human Rights and Charter of Fundamental Rights.
The State rejects all of his arguments and denies the 2011 legislation is incompatible with the Constitution, European Convention, or Charter of Fundamental Rights.
The outcome of this case is likely to have an effect on his appeal against his conviction, which has not yet been heard.
Dwyer is not in court for the hearing, which is expected to last around two weeks.