Graham Dwyer, who requested a retrial over phone data, was admitted into evidence

The challenge resulted in judgments in favor of Dwyer by the High Court, the Supreme Court and the Court of Justice for the European Union (CJEU).

The metadata proved crucial in establishing a connection between Dwyer and his victim, and also helped the detectives get a picture of their secret and abusive sadomasochistic relationship.

The evidence was included in Dwyer’s trial, despite opposition from his attorneys at the time.

The Court of Appeal heard today that Dwyer’s was no longer arguing for the evidence to be excluded entirely, but believed it should have been the subject of an admissibility test that arose from a Supreme Court case named JC.

Dwyer’s counsel, Remy Farrell SC, agreed with the President of the Court, Mr Justice George Birmingham, that the “high water mark” of his client’s position was that he wanted a JC type test and was not given one during the retrial .

In the JC case, decided shortly after Dwyer’s conviction, the Supreme Court narrowly ruled that unconstitutional evidence may be admissible, but only if prosecutors can show the violation was negligent.

Mr Farrell said his side must bring their case within the existing legal framework.

The lawyer told the court that ECJ rulings prior to the seizure of Dwyer’s metadata signaled that the system of “indiscriminate and general retention” was illegal in Ireland, where mobile operators are required to retain customer metadata for two years.

This view was reinforced by the ECJ in the lawsuit brought by Dwyer.

The issue of metadata is the first of several grounds of appeal addressed by the three-judge panel.

Dwyer, dressed in a navy suit, white shirt and navy tie, and with his graying hair cut straight, watched intently as his attorney spoke in court.

Dwyer’s father, Sean, watched from the bleachers, as did members of the O’Hara family.

Ms O’Hara, 36, a vulnerable woman who had received psychiatric treatment, went missing in August 2012.

Her remains were found 13 months later in a forest near Killakee in the Dublin Mountains.

Using the Communications Act 2011, Gardaí obtained data associated with Dwyer’s work cell phone and other handheld devices.

Using cell phone analysis, they were able to roughly determine where Dwyer was at any given time based on the movement of his work phone.

This helped investigators establish his routine in the months leading up to Ms O’Hara’s disappearance.

Cellular analysis was also able to link the movements of Dwyer’s work phone to those of a Nokia cellphone found in September 2013 in the Vartry Reservoir in Co Wicklow.

When Dwyer went somewhere, he usually brought both phones with him, he heard in court.

The charge was that this second handset, known as the “master” phone, was used by Dwyer to communicate with Ms. O’Hara.

She used another Nokia handset, known as the “slave phone,” which Gardaí procured during the investigation. Both the ‘master’ and ‘slave’ phones were discovered in Shankill, Co Dublin on the night Ms O’Hara disappeared.

Detectives were also able to “revive” text messages from phones and computers to reconstruct conversations between Dwyer and Ms. O’Hara.

The court heard that Dwyer only challenged the seizure of the metadata for his work phone. He denies owning or using the other phones.

Mr Farrell said the storage regime could be characterized as an “opportunistic form of mass surveillance” in which a phone is “converted into a sophisticated tracking device”.

He said the ECJ has repeatedly ruled that a system of general and indiscriminate retention of mobile phone data for law enforcement purposes is simply not allowed and that any retention or access must be targeted.

Mr Farrell said the ECJ clarified that the 2011 law, which was based on a 2006 EU directive that was later repealed, imposed obligations on mobile operators that breached EU law.

He said Dwyer’s case was “particularly important” because he was not under suspicion at the time his records were kept.

“With these records, Gardaí have created an extremely detailed picture of everything in Mr Dwyer’s life,” Mr Farrell said.

The attorney said the view that the law under which Dwyer’s data is stored and seized is problematic is not widely held.

He said the Supreme Court has clear reservations about targeted retention, believing it is not effective for detecting crime.

But Mr Farrell said whether one likes the ECJ’s rulings or not, they must be accepted and EU member states must “fully internalize the outcome”.

“All state institutions have a duty of loyal cooperation to the ECJ,” he said.

“This court is bound by the judgment of the Supreme Court and the ECJ.”

Gardaí obtained the metadata for the work phone from Dwyer for the period between October 7, 2011 and November 30, 2012.

Mr Farrell said the lengthy period helped underscore the “entirely indiscriminate nature of the surveillance”.

He also said that contrary to the prosecution’s claims, the metadata simply corroborated other evidence, prosecutors weaved the call data “into the carpet of evidence” and used it to “directly implicate” Dwyer.

“It’s hard to believe the only affirmative claim,” Mr. Farrell said.

Mr. Farrell said prosecutors argued that the trial judge’s discretion to admit the metadata evidence could only have been exercised in favor of admission. But he said he’s trying to demonstrate otherwise.

The court is scheduled to consider other grounds of appeal later.

These include alleging that the case should not have gone before a jury because no cause of death was determined and the prosecution failed to prove that Dwyer caused Ms. O’Hara’s death.

Other reasons include the playing of videos before the jury of Dwyer involved in sexual activity with women, which his attorneys said made a fair trial impossible for him, and the fact that Dwyer did not have his attorney present when he was questioned Gardaí in October 2013.

The appeal continues before Mr. Judge Birmingham, Mrs. Judge Isobel Kennedy and Mr. Judge John Edwards.