The Taoiseach says it is important that "no stone is left unturned" in the probe to bring Sophie Toscan du Plantier's killer to justice.
Gardai last week announced its Serious Crime Review Team will again look at the case of the brutal killing of the French filmmaker.
The 39-year-old mother-of-one was battered to death with a stone outside her holiday home in Toormore, near Schull in Co Cork on December 23, 1996.
Read More: Ian Bailey called on to 'come to France and clear your name' by family of Sophie Toscan du Plantier
And Micheal Martin told how he agrees with Garda Commissioner Drew Harris, who said the review is "not a futile exercise."
The Taoiseach said: "I don't believe it is optics. And knowing the commissioner as I do, and An Garda Siochana, this will be done professionally and objectively from a policing perspective.
"I do believe it is important that no stone is left unturned in terms of finding out and who murdered Sophie. And also bring that person to justice."
As part of the review into the near 26-year murder mystery, cold case detectives will use technology available now that wasn't a quarter of a century ago in a bid to establish who the killer is.
And the Taoiseach, who was speaking after the reopening of Fitzgibbon Street Garda Station today in Dublin's north inner city, added: "We do know with the advent of modern technologies that opportunities arise now that may not have been there at the time of murder."
Justice Minister Helen McEntee also explained that Commissioner Harris has assured her that "every effort" will be put behind this fresh probe.
And she said: "No cold case is opened with any intention other than to try and ascertain new evidence to try and ensure that justice is served.
"And in this instance to ensure that the person who was murdered Sophie Toscan du Plantier to justice."
Ian Bailey last week told how he hopes that there is justice for Sophie but also for him.
The Mancunian, 64, was arrested twice but never charged with the killing and has always denied any involvement.
Bailey was convicted in absentia of the murder in Paris in 2019 but the High Court here refused to extradite him.
However, legal experts have criticised the French trial in absentia which convicted Bailey, as much of it was based on the flawed original Garda file.
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