On August 4, two Cuvier's beaked whales were found dead in Sheephaven Bay and Gola Island.
However a third whale has now been found washed up Pollan beach in Ballyliffin in recent days.
A Derry holiday maker who saw the whale said it was a very sad sight.
"I was on the beach with my son and my husband and we came across the whale," she said.
"It was very upsetting to see such a wonderful mammal lying dead like that."
The Irish Whale and Dolphin Group (IWDG) has described the strandings as "highly unusual" for the deep-diving mammal.
Cuvier's beaked whales are a deep diving species and have previously been found to be particularly affected by loud underwater man-made noise (eg sonar).
Comment: See also: Three dead beaked whales found in 'highly unusual' strandings on west coast of Ireland