I have just watched the Killarney lake video on the blog; a very interesting looking film. I can certainly see what you mean about the giant eels; highly plausible but the mystery animal at 6.44 onwards I have my doubts about.
I have watched it a number of times and every time I see it, it looks less and less like a lake monster. Actually, it looks less and less like an underwater creature.
Being honest, I would say it is a large water bird, which appears to be taking off; as you watch the object move across to the right you can see what appears to be a flapping motion and the animal make contact with the water on a few occasions.
I would suggest from this image that it is either a large bird taking off or an osprey successfully hunting;
the reason being that the only reason for a fish to break the surface hunting would have to be for insects and it would then require a bloody big fly to attract a creature that size out of the water. It would not, however, be to hard to imagine a bird catching a large fish on the surface that in turn was hunting smaller fish, that in turn are eating the flies.
Interesting lake, mind you
all the best
I have to admit that the more I look at the video I think that you are partly right. The black dot does appear to be a bird, but the more I examine both the video (at as high resolution as possible) and the photographs that my darling Corinna took, the `torpedo` wake seems to be following the bird rather than being made by it. The bird is actually a `red herring` (if I may mix my metaphors into a gloriously surrealchemical game of verbal exquisite corpse), because the `torpedo wake` bears off to the right as the bird (probably a cormorant or a grotesque shag) flaps on its merry way. Maybe whatever it was in the water was chasing the bird?