One more pic added to show the shape of the dorsal fin.
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9m is at the upper end for adult minke but they tend to have white 'armbands' on the pectoral fins and we can clearly see that's not the case. As you say, a young fin whale would have had asymmetrical coloration on the jaws and baleen, so this looks like sei. Very sad that it's stranded but good to know they're around and breeding.
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Yes thats what Ian Enlander said about the Minke also, white stripe on the pectoral in most cases. It does appear to be a Sei going by the dark evenly coloured baleen and the nature of the dorsal fin.
Ian says that he's observed 24 species of whale and dolphin in Irish waters but that he'd never seen a live Sei Whale to his knowledge. He did see another stranded one many years ago coincidentally enough also in Co Antrim, about 20 miles to the south at Islandmagee. I'll double check that.
The extra photo showing the dorsal fin shape is very useful, because the other species this could have been is blue whale (although it would have to have been very young at that size). Blue whale coloration does vary enormously, and they often have blotches like this animal's, but the dorsal fin is a tiny, stubby thing. This individual has a very defined, falcate dorsal fin. The circular marks on sei whales are usually made by cookie-cutter sharks or lampreys - if the former, this would suggest the animal had spent some time further south, perhaps off Spain or towards the Azores.
I can't claim to have even heard of Sei Whales unti now - the only reason I have not agreed.
...here a stranded Minke in Kent for comparison
Not nice to see these majestic animals in this state
Lat/Lng: 55.0664, -6.0545
OS grid ref: NW412821