The European Court of Justice has ruled in favour of Graham Dwyer's arguments against the legality of a law, governing the retention of phone traffic and location data.
The Court of Justice of the European Union says EU law prevents the 'general and indiscriminate retention' of traffic and location data for the purposes of combating serious crime.
It says: "While the privacy and electronic communications directive allows member states to place limitations on the exercise of those rights and obligations for the purposes inter alia of combating crime, those limitations must comply with the principle of proportionality".
And it adds: "The court has already held that the objective of combating serious crime, as fundamental it may be, does not, in itself, justify that a measure providing for the general and indiscriminate retention of all traffic and location data".
The Supreme Court will now have to uphold a previous legal win Dwyer had in the High Court, in a move which is likely to strengthen his case when he eventually appeals his conviction for the murder of Elaine O'Hara.
In 2015, Dwyer was handed a life sentence for her murder.
The year before his conviction, the EU directive underpinning Irish law which allowed Gardaí access his mobile phone data was struck down.
In 2018, the High Court ruled that the law was "too general and indiscriminate" when it came to the retention of that information.
Given the potential consequences when it comes to the investigation of serious crimes, the State appealed that decision to the Supreme Court - which requested clarity from Europe on a number of issues.
In November, the legal adviser of the European Court of Justice backed the 2018 judgement of the High Court, and also referenced the fact that access to such data does not appear to be subject to prior review by a court or an independent authority.
Additional reporting: Jack Quann