The state's appeal against a legal challenge won by Graham Dwyer will be heard on Monday.
The supreme court will sit in Waterford for the first time and it is being broadcast live on TV.
A high court ruling in 2018 found part of the legislation allowing mobile phone data to be retained and accessed, breached EU law and the European Convention on Human Rights.
Graham Dwyer was found guilty of murdering Elaine O'Hara following a high profile trial in 2015.
Image: Graham Dwyer