it sorta sums up the tune in some ways

Except that nobody said the tune originally had anything to do with Bunessan. The connection is with the words to an entirely different hymn that uses the same tune as "Morning has Broken".

I've been to Bunessan several times. Not a hugely exciting place, though the local pub used to have very good ceilidhs - I'm not sure if they still happen. Facing north with the hills behind it, it can be bleak and gloomy in winter.

There are quite a few American sites that claim some of Eleanor Farjeon's work as out of copyright (one of her children's books, republished by Project Gutenberg). Since she only died in 1965, that is certainly not the case in many other countries. If US law classifies "Morning has Broken" as public domain in the same way, Americans are getting something for free that would otherwise be earning very substantial sums for Farjeon's estate in the UK. American copyright law can be as sleazy as the US's idea of "free trade".

The tune seems more similar to "Air Fa-La-La-Lo" than anything else to me, though I can sort of see a very distant similarity to "The Battle of the Somme".