A mass beaching of 35 long-finned pilot whales off Co Donegal has been confirmed by the Irish Whale and Dolphin Group (IWDG).
A chain of the dead whales was discovered on the high tide mark on Saturday afternoon on a beach on Rutland Island. The group - mostly mothers and calves - died near each other on the sandy shoreline.
Experts believe it is the same group being monitored off South Uist about ten days ago. On Tuesday, they reappeared, about 100 miles to the south, off the north Irish coast.
Dr Simon Berrow, of the IWDG, fears naval sonar equipment could have disturbed the whales' navigational systems.
He said his organisation had "contacted veterinary colleagues in the UK, who are particularly interested to establish the cause of death.
He said it might be the same group as the whales at risk of a live stranding in South Uist.
He added: "Plans to carry out a post-mortem examination have been cancelled, given the severe weather forecast over the next few days. A team from the Galway-Mayo Institute of Technology and IWDG are planning to record length, gender and obtain photographs to see if we can match them to the Scottish animals.
"Skin samples and teeth will also be collected for genetics and life-history studies," he said.
The Scottish SPCA, British Divers Marine Life Rescue (BDMLR) and Western Isles Coastguards were poised to mount a rescue operation when the pod was previously in danger of dying in a South Uist sea loch.
The whales were reported to be in distress and circling close to the shoreline in Loch Carnan.