Irish citizen Paul Whelan is one of several people who have been released as part of a major prisoner swap involving the US and Russia.
Mr Whelan, a former US marine, also holds US, Canadian and British passports.
He was arrested in December 2018 and accused of being caught while on a 'spy mission'.
In the biggest such exchange since the Cold War, a large number of individuals have been freed - including Wall Street Journal reporter Evan Gershkovich.
Taoiseach Simon Harris has thanked all those involved in Mr Whelan's release.
"I want to welcome the release of Irish citizen Paul Whelan from prison in Russia," he said.
"I have received confirmation from the US government that Mr Whelan was part of a prisoner exchange earlier today negotiated over many months.
"I have spoken to the US Ambassador in Ireland who confirmed the news.
"I know this will be an enormous relief to his family who have campaigned for many years for his release.
"I would also like to thank Ireland’s diplomatic team in Russia and our consular team in Dublin for their years of tireless work on this case," he added.
'Joyous day'
Some two dozen people from countries including Russia, the US, Germany, Poland, Slovenia, Norway and Belarus have been moved.
Among those being released from Western prisons is Vadim Krasikov, a Russian hitman serving a life sentence in Germany for the 2019 killing of a Georgian citizen in Berlin.
US President Joe Biden will meet the families of the released Americans at the White House on Thursday.
The complex trade was negotiated with Russia and several other countries in secret for more than a year.
In a letter, Wall Street Journal editor-in-chief Emma Tucker said it was a "joyous day" after the release of her reporter, Mr Gershkovich.
She added: "That it was done in a trade for Russian operatives guilty of serious crimes was predictable as the only solution given President Putin's cynicism."
Mr Gershkovich (32) was arrested and detained in March 2023 after Russia claimed he had been "gathering secret information" on orders from the CIA.
He said the charges against him were false and his employer called the case a sham.
