Mullingar security analyst Declan Power says nobody knows why the vessel is there.
The Defence Forces are monitoring a Russian spy ship spotted off the west coast.
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, Mullingar's Declan Power says nobody can know for sure why the vessel is there, unless they board it and ask:
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