Killer Graham Dwyer's bid to overturn his murder conviction will be delayed for up to two years after the Supreme Court referred key legal questions on the retention of communications data to a European court.
The referral will also delay the introduction of a new legislative regime for gardai to access such data.
Hundreds of investigations have been stifled since Dwyer secured a High Court declaration in 2018 that legislation used by gardai to access his phone data contravened EU law and the European Convention on Human Rights.
The data was crucial to proving he murdered childcare worker Elaine O'Hara.
Dwyer wants to rely on that ruling in a separate challenge to his conviction, where he intends to argue the data should not have been admitted in evidence at his trial.
A jail source told the Herald that Dwyer seemed "happy out" after learning of the decision.
In a significant judgment delivered by Chief Justice Frank Clarke yesterday, the Supreme Court upheld the High Court ruling that the manner in which gardai accessed retained phone data did not meet EU standards due to a lack of independent oversight.
However, he also said he believed an Irish court making a finding that an Irish law was inconsistent with EU law had the power to decide any such invalidity was not retrospective.
This means such a decision would only apply from the date of the decision, ruling out the prospect of Dwyer being able to rely on it as a ground of appeal.
But the court will not finalise its ruling until three key questions have been considered by the Court of Justice for the European Union (CJEU).
It is understood that Dwyer watched proceedings, which were screened live on RTE, from his cell in the Midlands Prison.
"There was no major excitement out of him - he does not really interact with many staff or other inmates," a jail insider told the Herald.
"But it was fairly clear that he was happy out with the result as he looked content and was smiling a lot," the source added.
While the average waiting time for a referral to be processed is around 15 months, lawyers say it can take between 12 months and two years.
Dwyer (47) was jailed for life in 2015 for the murder of Ms O'Hara (36), who was last known to be alive on August 22, 2012. Her remains were discovered at Killakee in the Dublin Mountains 13 months after her disappearance.
In a statement, Justice Minister Charlie Flanagan said it was clear the current legal framework needed to be modernised.
He said drafting of legislation to replace the legislation at issue, the 2011 Communications Act, was "well advanced", but now needed to take account of the CJEU referral.
In the absence of new legislation, gardai have no other means for lawfully accessing communications data.
Yesterday's decision came after the State appealed the 2018 High Court ruling. The key issue highlighted by the High Court was a provision which allowed gardai of chief superintendent rank and above to request user data from providers.
Mr Justice Tony O'Connor found the data retention regime was general and indiscriminate, with no prior review by a court or an independent body and no adequate legislative guarantees against abuse of such data.
In its ruling, the Supreme Court found there must be independent prior permission given for such access.
"I express the view that the Irish access regime does not meet that standard, not least because of the fact that, albeit access permission is granted by a separate unit within the force, such permission is nonetheless granted from within An Garda Siochana so that there is insufficient independent review in advance of the need for access," Mr Justice Clarke said.
He said the Supreme Court was obliged under EU treaties and case law to make a referral to the CJEU on an issue of EU law unless there was already a "clear" answer to the issue.
Sources in the prison where Dwyer is serving his life sentence said the architect had consistently predicted to other inmates that his case would ultimately be decided in Europe.