The Defence Forces scrambled to deploy ships and aircraft this week after spotting two Russian government vessels off the west coast equipped with technology capable of interfering with subsea cables.
Suspicions were raised when the ships were spotted loitering near a newly operational communications cable linking Ireland and Iceland.
Both ships had been involved in the construction of the Nord Stream 2 pipeline, resulting in them being sanctioned by the US government.
After being spotted earlier this week, the ships were put under surveillance by Air Corps aircraft and Naval Service assets. However, later analysis determined the ships’ unusual movements were probably a result of efforts to avoid bad weather, rather than anything sinister.
Both ships left the Irish Exclusive Economic Zone (EEZ) on Monday and are currently sailing south to the port of Malabo in Equatorial Guinea on the west coast of Africa. They departed the northern Russian port of Murmansk three weeks ago.
They remained under surveillance during their time in the Irish EEZ although it is understood this surveillance was not constant due to poor weather and a lack of suitable aircraft.
EU countries, including Ireland, have stepped up surveillance of vital subsea infrastructure since the explosions that disabled the Nord Stream pipelines last year. The culprits have not been identified, although the US, Russia and Ukrainian groups have all been accused of being behind the attack.
The two Russian vessels, the Umka and the Bakhtemir, raised serious concerns among Irish military officials due to their movements around the IRIS high-speed, subsea communications cable, which became operational last year and runs west off the Galway coast.
The ships appeared to double back on themselves several times in the general area of the cable.
Suspicions were heightened after an Air Corps aircraft observed one of the ships towing a large platform. This was later confirmed to be a 100m long cable-laying barge.
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The Umka is an offshore supply vessel and the Bakhtemir is classified as a salvage and rescue ship equipped with diving platforms and high tech submersibles capable of deep water work on infrastructure.
Both are flying the Russian flag and are operated by the Russian Marine Rescue Services, which was also sanctioned by the US for its involvement in Nord Stream 2.
The Defence Forces confirmed it was involved in an operation but declined to give further details.
A spokesman said “both the Air Corps and the Naval Service are aware of the ships but we won’t be releasing any further information on the operation.”
He added that both services “continue to monitor activity in Irish waters and to undertake Maritime Defence and Security Operations (MDSO) throughout Ireland’s maritime domain.”
