'Summer is acome unto Day' is clearly a corruption of 'Summer is y-comen today' (y-comen being an old form of 'has come').

I reckon 'Aunt Ursula Birdhood' is really something to do with Saint Ursula - a martyred Westcountry Romano-Celtic virgin. She was a princess & daughter of King Dionotus. (Could he be one & the same as King Doniert, I wonder?)

Reference to 'an old ewe' in a song about the return of Spring sounds, to me, like a garbled version of something about 'old anew'.

Any Cornish or Middle-English speakers have any ideas about meanings of the lyrics?