An advisor to the European Court of Justice (ECJ) is of the opinion that the 'general and indiscriminate' retention of mobile phone data is unlawful.
If adopted by the highest court in Europe, his stance will greatly boost convicted killer Graham Dwyer's legal challenge to the controversial law.
In April 2015, Dwyer was found guilty of murdering childcare worker Elaine O'Hara.
Mobile phone data played a crucial role in the prosecution's case - not just content, but also the tracking of relevant handsets through cell-site analysis.
Three years ago, he won a significant challenge against the law which allowed that data to be retained and accessed.
The High Court felt its retention was "too general and indiscriminate".
The State appealed that decision to the Supreme Court, which in turn sought clarity on a number of issues from the European Court of Justice.
In what could be a significant development, an advisor to the ECJ gave an opinion which supports the 2018 High Court ruling.
His opinion is not binding on the court, but it is seen as a good litmus test for the judgement that will follow in the coming months.
If the Supreme Court upholds the High Court's judgement, then Dwyer will feel like he has a much stronger hand when he eventually appeals his conviction.