I haven't seen any mention in the above discussion that

a) We are talking about two things, a tune and a song

b) Various different lyrics have been set to the tune

c) The tune is a 3/4 (slowed down) member of a large tune/song family, most of which are in 6/8.
For one well known example, we have 'Shepherd oh Shepherd'

d) The Morris Dance tune Greensleeves is also related, more distantly