The UK had to come to Ireland's aid as a Russian submarine was "hovering" just 12 miles away from Cork Harbour.
The incident happened six months ago, and details have been emerging.
It is reported the British Royal Navy sent a helicopter to drop sonar technology into the water, before sending an anti-submarine boat.
Security and defence analyst Declan Power said Britain often finds out about these things before we do.
Mr Power told Newstalk: "What exactly was the Russian submarine doing there? It should be looked at in the broader array of defence arrangements in that the Russians will be regularly testing the defence responses of Nato nations... in particular the UK.
"Because the UK and the Scandinavian countries have responsibility for monitoring an area known as the Icelandic gap."
The incident with the Russian submarine occurred directly south of the entrance to Cork Harbour, according to the Irish Examiner.
Russia has been regularly testing British air defences off Irish shores in recently years.
