The European Court of Justice is expected to give an opinion later on the challenge taken by Graham Dwyer over the retention and accessing of his mobile phone data.
Dwyer was convicted in 2015 of the murder of Elaine O'Hara in August 2012.
Mobile phone metadata obtained by Gardai played a significant role in the trial, however Dwyer claims the legislation that was used breached EU law.
Today's opinion by a senior judicial adviser will be non-binding but it could have significant influence on the ECJ's final judgement.
