Lawyers for convicted murderer Graham Dwyer will go to the High Court tomorrow to begin his action against the Garda Commissioner and the State over the collection and retention of mobile phone data.
Data from mobile phone companies played a central part in Dwyer's trial and conviction three years ago for the murder of Elaine O'Hara in 2012.
His lawyers will 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.
The information, which was obtained from mobile phone service providers, allowed the prosecution to show Dwyer's mobile phone had been in certain places at certain times and allowed them to see where he was when calls and texts were made and received.
Gardaí were able to get this information under 2011 legislation brought in to give effect to a European directive.
The legislation obliged service providers to hold on to mobile phone data for two years.
However, the directive itself was struck down by the Court of Justice of the European Union in 2014 as an unjust attack on the right to privacy and data protection.
The admissibility of the data was challenged by Dwyer's lawyers at his trial but allowed in by the judge.
The outcome of this case is likely to have an effect on Dwyer's appeal against his conviction, which has not yet been heard.
Dwyer was jailed for life in April 2015 after a jury unanimously found him guilty of murdering Ms O'Hara in August 2012.
Her remains were found in the Dublin mountains just over a year after she was last seen.
Her keys and other items were found in Vartry Reservoir in Roundwood three days later.
Dwyer is not expected to be in court for the hearing, which is expected to last around two weeks.