Russia has retaliated over the expulsion of one of its Ireland-based diplomats by expelling by an Irish one from Moscow.
Confirming the anticipated move by the Kremlin, the Government branded it unjustified and regrettable.
Earlier this week, Ireland ordered a Russian diplomat to leave the embassy in Rathgar, Dublin.
Taoiseach Leo Varadkar said Ireland was showing solidarity with the UK in the wake of the Salisbury chemical attack on former Russian intelligence officer Sergei Skripal and his daughter Yulia.
On Friday, Tánaiste and foreign affairs minister Simon Coveney confirmed Russia had reacted in kind.
A spokesman for Mr Coveney said: "There is no justification for this expulsion.
"Our staff do not engage in activities which are incompatible with their diplomatic status. This decision to expel an Irish diplomat is regrettable."
After taking action against Russia, Mr Varadkar insisted he had not undermined Ireland's neutrality.
Some opposition parties in the Dáil, including Sinn Féin, heavily criticised the diplomatic sanction imposed by the Taoiseach.
Russia's ambassador to Ireland, Yury Filatov, also criticised the expulsion of one of his country's diplomats.
"This kind of decision is totally unwarranted, uncalled for, senseless and regrettable," he said.
One Irish diplomat has been instructed to leave Russia the Department of Foreign Affairs has confirmed.
The Kremlin is responding after a group of western countries, including Ireland, supported Britain in the wake of the Salisbury spy poisoning.
A spokesperson for the Tánaiste says there's no justification for the "regrettable" decision as Irish staff do not engage in activities which are incompatible with their diplomatic status.
Meanwhile, it is understood the condition of Yulia Skripal is improving dramatically, and she is now eating and drinking.