The oil was spotted near the Russian vessel, top left.
The Russian navy says its vessels were to blame for an oil spill in the Celtic Sea off Ireland about 10 days ago.
An investigation found "technical malfunction and human error" resulted in hundreds of tonnes of oil leaking from a tanker anchored near a warship.
An Irish Department of Transport spokeswoman said the spill had broken up significantly since it was first spotted on 14 February.
It was still being monitored, 45 miles south of Ballycotton, County Cork.
It was first spotted by aerial pollution surveillance on 14 February, and was estimated to be between 300 to 1,000 tonnes.
Irish coastguards said the spill, 50 miles (80km) off west Cork, covered 2.8 miles by 3.1 miles around a Russian aircraft carrier and a tanker.
"It has deteriorated significantly now," the spokeswoman said on Thursday, adding that it had not reached any coastline.
Two refuelling tankers, one aircraft carrier, one Russian tug, a Russian destroyer, a British destroyer and an Irish naval vessel were in the area at the time of the incident.