The Yantar vessel is capable of attaching spy equipment to undersea cables that carry internet traffic, according to The Irish Times.
It carries a crew of 60, and has also been deployed in the past for search and rescue missions.
Security analyst Declan Power says nobody can know for sure why the vessel is there, unless they board it and ask.
Mr Power said: “Based on the fact that there has been a certain amount of activity off the west coast where we have cables that carry internet traffic between Europe and the Atlantic, it is suspected that the Russians are showing some undue attention to that area and this has been alleged before with other circles.”
The Defence Forces told The Irish Times it was aware of the presence of the vessel off the west coast of Ireland.
“Óglaigh na hÉireann can confirm that the Irish Naval Service is aware of the Russian ship Yantar transiting in the Irish exclusive economic zone off the west of Ireland,” it said.
“This ship is transmitting on the automatic identification system and is outside Irish territorial waters.
“This activity is in line with the UN Convention on the Law Of the Sea (UNCLOS) rules for transit through international waters.
“Territorial waters are 12 nautical miles from the coast of Ireland and under UNCLOS, there is no restriction on ships operating on the high seas inside exclusive economic zones.”
