Irish animal welfare volunteers are on alert after a large pod of pilot whales at risk of a mass stranding in the outer Hebrides headed south towards Ireland.

A rescue operation swung into action on South Uist last Thursday after more than 60 of the mammals swam into a narrow and rocky sea loch on the island's coast, prompting fears that dozens could be killed in a mass beaching. Two were found dead as the pod left Loch Carnan, on the north-east corner of South Uist on Sunday.

The pod's departure has raised fears that the whales could soon arrive off the Irish coast and again risk stranding in a repeat of an incident off Ireland last year. A similar alert was raised in Loch Carnan in October after 35 pilot whales appeared to be at risk of becoming beached. They had left South Uist intact but less than a week later 33 of the pod were found dead on a deserted island off County Donegal.

Mick O'Connell, strandings co-ordinator of the Irish Whale and Dolphin Group, said it was braced for a similar incident.

"We're aware there's a reasonable chance of it happening again, but it might not, every single one is different. We're relying on our members and the public to report if they see them," he said.

Simon Berrow, the group's overall co-ordinator, said: "We're on standby to see if the pilot whales come to Ireland, and we're getting regular updates from our colleagues in the UK. So we're watching and waiting."

A postmortem on the first dead whale found on Saturday showed it was a lactating female with a disease in its melon, a fatty organ in its forehead. Welfare groups hoped its calf would be strong enough to survive on it own, but were reassured that the whale did not die after attempting to strand itself or from colliding with rocks on Loch Carnan.

The second dead whale was spotted in the loch on Sunday, but the weather conditions were too severe for its body to be recovered.

Dave Jarvis of the welfare group British Divers Marine Life Rescue, which co-ordinated last week's rescue operation, said he did not believe the whale died in the loch. "Due to its location and orientation it is thought that the creature probably died elsewhere in the area and floated in on the incoming tide. It is currently impossible to comment on why this animal died," he said.