Ireland opened the investigation six months ago after the FBI smashed a Russian spy ring of 11 men and women posing as American civilians, including 28-year-old Anna Chapman.
Several of the spies were found to have used Irish passports as part of their travels to and from Russia and other countries.
The department of foreign affairs, in Dublin, said police had concluded that Russian agents stole the personal details of six Irish citizens and used them to counterfeit Irish passports.
The US expelled 10 spies to Russia in July in exchange for four people convicted in Russia of spying for the west.
Most of the Russian spies had been living in the US since the 1990s and had instructions to work their way into influential business and political circles, but they largely failed in that mission. Their fabricated identities included surnames common in Ireland, including Murphy and Foley.
The department of foreign affairs said a senior Irish official had issued a face-to-face reprimand to Russia's ambassador to Ireland, Vladimir Rakhmanin.
It said the ambassador had been told: "The activities of Russian intelligence services in connection with the forgery of Irish passports and the effective theft of the identity of six Irish citizens are completely unacceptable and not the behavior the [Irish] government would expect from a country with which we have friendly relations."
It declined to identify the Russian embassy official being expelled or to specify whether the official was directly linked to the theft or counterfeiting efforts.
"It is regrettable that this action has been necessary. However, the primary responsibility of the government is to ensure the security and wellbeing of Irish citizens, which includes protection of the integrity of Irish passports," the department said.
