The nine-metre long Sei whale was discovered in Waterfoot, Co Antrim. It died from suffocation, an expert said.
Ian Enlander, from the Irish Whale and Dolphin organisation, said: "It was probably travelling with its mother and has become separated."
It is commonly found in the deep waters of the Atlantic.
Mr Enlander said: "It probably became disorientated and became trapped."
COMMENT RULES: Comments that are judged to be defamatory, abusive or in bad taste are not acceptable and contributors who consistently fall below certain criteria will be permanently blacklisted. The moderator will not enter into debate with individual contributors and the moderator’s decision is final. It is Belfast Telegraph policy to close comments on court cases, tribunals and active legal investigations. We may also close comments on articles which are being targeted for abuse. Problems with commenting? firstname.lastname@example.org