Tánaiste Micheál Martin has passed on his sympathies to the family of Finbar Cafferkey who was killed in eastern Ukraine.
Mr Martin said he was a man of "clear principles".
"My deepest sympathies to the Cafferky family on the sad passing of Finbar," Mr Martin said.
"He obviously was a young man of clear principles.
"Our thoughts and our prayers are with the family today.
"I think the family have asked for privacy, I'm not going to comment any further on it at this particular time.
"I'll allow the family space to grieve what is a very, very sad loss for them."
Mr Cafferkey is understood to have travelled to the country to assist local forces.
He had previously fought with Kurdish militia in Syria, during their engagements against IS.
Mr Cafferkey was in his 40s and from Acaill (Achill Island).
At this stage, there are no details relating to the circumstances in which he died.
The Department of Foreign Affairs said it is aware of the case and is providing consular assistance.
Russian embassy critical of Martin's comments
The Embassy of Russia in Ireland issued a statement this evening responding to Mr Martin's comments.
The embassy took issue with Mr Martin referring to Mr Cafferkey as "a young man of clear principles".
"We do not know what his principles were," said the embassy.
"What we do know, though, is that in a very big way it is the Irish Government and media who bear responsibility for the death of Finbar Cafferkey.
"It has been the Government and media who have been promoting anti-Russian propaganda, distorting the truth about the conflict in Ukraine, misleading people like Finbar Cafferkey. Now they face the results of their own efforts," added the statement.
