possibly a total coincidence, one of those 'happy accidents,' but it cracks me up that úll is a homophone to 'ool,' the word for 'food' in Ringo Star's 1981 movie, 'Caveman.' i always remember 'úll' means 'apple' cause i always think of the cavemen in that movie eating those fruits that looked like mangos, and saying 'ool.'
I did get the impression from the voice in Duolingo. I rechecked on Foclóir. None of the three dialects seem to have a "Yu" at the beginning. All are identical between úll and Iúil. With the latter half there are differences between the dialects, but I couldn't really notice a system behind it. Like the Connact "l" of Iúil sounds absolutely broad and totally homophone to Munster úll. Connacht úll on the other hand sounds like OO-le, something which I would expect of a slender l.
I guess I shouldn't dwell too much on it. I hope I get it in time. (although I have to say the dialect stuff IS interesting)
OTOH, I will probably never speak with anybody in Irish so the point is rather moot.
An easy way to remember this month is by looking at the etymology of "July". It comes from Latin Iulius, named for the Roman emperor Julius Caesar after his death and deification. The letter J was originally just a modified I and was pronounced the same -- a tradition that's continued in most Germanic languages, and the J/I similarity is reflected here in Irish.