President Michael D Higgins has called for clarity around the death of Alexei Navalny.
One of Russian President Vladimir Putin's most prominent critics had been convicted of charges that were widely considered to be political in nature.
The 47-year-old was sentenced to 19 years behind bars in 2023 and moved to one of Russia's toughest prisons, deep within the Arctic Circle.
The country's prison service said Mr Navalny had "felt unwell" after a walk today and "almost immediately lost consciousness".
They claimed efforts to revive him failed and are now trying to establish a cause of death.
President Michael D Higgins is seen in Co Wicklow in October 2021. Image: Damien Eagers / RollingNews.iePresident Higgins said facts around his death must be made available.
"The freedom to express dissenting views, be it on the structure or administration of society, is a fundamental tenet of a democracy and any accountable system," he said in a statement.
"The incarceration of Alexei Navalny contradicted this.
"It is important that all of those who believe in these principles support the making available of all of the facts surrounding Alexei Navalny's death," he added.
Earlier EU Council President Charles Michel said Mr Navalny had made the "ultimate sacrifice" for his ideals.
Mr Michel said the EU holds the Russian regime solely responsible for his death.
Last year another Putin critic, Wagner boss Yevgeny Prigozhin, also died suddenly.
Mr Prigozhin helped organised an aborted coup in June and died in a plane crash two months later.
