Graham Dwyer is hoping for a significant boost from a European Court of Justice decision on the retention of mobile phone data today.
It could greatly help his appeal against his conviction for the murder of Elaine O'Hara.
In April 2015, architect Dwyer was found guilty of murdering vulnerable childcare worker Elaine O'Hara nearly three years earlier.
Mobile phone data played a crucial role in the prosecution's case against him.
But over three years ago in the High Court, he won a significant challenge against the law which allowed that data to be retained and accessed.
The state appealed that to the Supreme Court, which in turn sought clarity on a number of issues from the European Count of Justice.
Last November an advocate-general to the ECJ gave an opinion which supports the 2018 High Court ruling.
A final judgment is due today - and barrister Ronan Lupton expects a boost for Dwyer.
"The Court of Justice of the European Union follows the advocate general's opinion in about 75% of cases.
"I think that there's a strong chance that the advocate general's opinion handed down last November will be followed by the Court of Justice in its ruling today in relation to the Graham Dwyer appeal."
Lupton though says Dwyer would still face a big battle to overturn his conviction.
"Not all the evidence gathered in respect of Mr Dwyer's case was that of mobile phone data retained, which may have been unlawfully retained depending on what the Court of Justice finds in its ruling later today."
The ECJ will give its judgment in Luxembourg at 9 o'clock this morning, which is 8 o'clock Irish time.
