Alexei Navalny's death "underpins the lack of respect for the rule of law" in Russia, Tánaiste and Minister for Foreign Affairs Micheál Martin has said.
In a statement, Mr Martin said he is "deeply shocked" by the announcement and said it is a reminder of the "repressive nature" of the Russian regime.
He added: "Ireland had consistently called for Mr Navalny’s unconditional release before his death."
Ukraine's President Volodymyr Zelensky said it is "obvious" that Mr Navalny was killed by the regime of President Vladimir Putin.
"Obviously he was killed by Putin. Like thousands of others who have been tortured," said Mr Zelensky, adding that this demonstrates why President Putin must be made to "lose everything and held accountable for his actions".
NATO chief Jens Stoltenberg said there were "serious questions" for the Kremlin to answer over the death.
Navalny has paid with his life for his "resistance to a system of oppression", French Foreign Minister Stephane Sejourne has said.
"His death in a penal colony reminds us of the reality of Vladimir Putin's regime", said Mr Sejourne
Norway's Foreign Minister said Moscow bears a "heavy responsibility" over Navalny's death.
"The Russian government bears a heavy responsibility," Foreign Minister Espen Barth Eide wrote on X, adding that he was "deeply saddened by the news".
Meanwhile, Russian newspaper editor and Nobel Peace Prize laureate Dmitry Muratov told Reuters that Navalny's death was "murder", and said that he believed prison conditions had led to his demise.
Russia's Investigative Committee has launched a procedural probe into the death, the Investigative Committee said.
The Latvian President said that Navalny had been "brutally murdered by the Kremlin".
"Whatever your thoughts about Alexei Navalny as the politician, he was just brutally murdered by the Kremlin. That's a fact and that is something one should know about the true nature of Russia's current regime," President Edgars Rinkevics said on X, offering condolences to Navalny's relatives.
