Ian Bailey has been ordered to pay all of the legal costs of his failed High Court action - estimated at up to €5m.
He had sued the State over the conduct of the garda investigation into the late 1996 murder in west Cork of Sophie Toscan du Plantier.
A stay on the costs order applies should Mr Bailey appeal the rejection of his case by a jury last March.
Any appeal must be lodged within 28 days of the costs order being finalised and legal sources consider an appeal is likely.
In his costs ruling, Mr Justice John Hedigan dismissed as "unreal" Mr Bailey's argument the Garda Commissioner and State should pay about half of the costs.
Mr Bailey claimed this was because the State only applied on day 60 of the 64 day case to have various claims, including wrongful arrest, not go before the jury on grounds they were brought outside the applicable six year legal time limit (Statute of Limitations).
The "central plank" of Mr Bailey's case, his claim that gardaí conspired to frame him for murder by threatening or coercing Marie Farrell into making false statements, went to the jury, the judge said.
It was incorrect to say most of the case was withdrawn, he said.
While it was "unfortunate" the case took so long, the judge said he was satisfied it was necessary.
That was not just in the interest of gardaí but the public interest, to have a full ventilation of the grave and very serious allegations made by Mr Bailey, and for gardaí to have a full opportunity to answer those.
Mr Bailey had wanted his case heard in open court and there would have been an "outcry" if it was closed down earlier on a "technicality", the judge said. It was also clear gardaí came forward to vigorously deny the claims made against them and they were entitled to their day in court.
Even if the State made the application under the statute of limitations earlier, he would have been conscious this was a case that required to be fully heard such was the depth and breadth of the allegations made, the judge said.
The public interest "deserved nothing less".
There were no special circumstances requiring the court to depart from the normal rule that costs go to the winning side, the judge found.
He made a full order for costs against Mr Bailey, including costs of discovery of documents in advance of the case and half the costs of a juror's cancelled holiday.
Mr Bailey was in court for the ruling.
His case opened last November and concluded after 64 days on March 30 when the jury dismissed his claim gardaí conspired to frame him for the murder of Ms du Plantier, whose body was found near her holiday home at Toormore, Schull, on December 23, 1996.
Her murder has never been solved.