The Supreme Court will give its decision on Monday in the State's appeal against convicted murderer Graham Dwyer's successful challenge to the retention and accessing of his mobile phone data.
The seven judges will give their decision in Waterford Courthouse.
The appeal, heard by the court before Christmas, was told that there were "enormous amounts" at stake in the case.
The State appealed the High Court's 2018 decision, which found part of the legislation allowing mobile phone metadata to be retained and accessed as part of the investigation of serious crimes, breached EU law and the European Convention on Human Rights.
The data played a significant role in the successful prosecution of Dwyer in 2015 for the murder of Elaine O'Hara in 2012.
The prosecution was able to show where Dwyer's phone was at certain times and what numbers it contacted.
Gardaí were able to get this information under Irish legislation brought in following a European directive, which was later declared invalid.
The High Court found the legislation allowing such data to be retained was general and indiscriminate and breached EU law.
It said the Court of Justice of the EU had ruled that even the objective of fighting serious crime, could not justify such an indiscriminate regime of retaining data.
The High Court ruled the system of accessing the data breached the European Convention on Human Rights because there was no prior review by an independent authority before it could be accessed.
The State appealed and the appeal went directly to the Supreme Court because of the importance of the legal issues involved.
Lawyers for the State told the court in December that the case raised fundamental questions about telecommunications metadata.
They said there were enormous amounts at stake, not only in relation to Dwyer's conviction but in relation to the powers of the State in retaining metadata and accessing it in the context of the fight against serious crime.
Senior Counsel Paul Gallagher said if the High Court judgment were to stand, it would mean there was a virtual space in which criminality existed and that could not be accessed by State authorities.
Dwyer's appeal against his conviction is on hold pending the outcome of these proceedings.
His legal team argued that the legislation allowing data to be retained was general and indiscriminate and there was no independent oversight of the accessing of such data.
They said the High Court ruling should be upheld.
Both sides agreed that if there was a lack of clarity in previous judgments of the Court of Justice of the EU on the issues, the Supreme Court should make a reference to the Court of Justice of the European Union.
