Reddit mentions of The Fairy-Faith in Celtic Countries (Library of the Mystic Arts)

it's always interesting to me that quite consistently people see female beings/spirits/whatever wearing white. you didn't say whether your father felt this being to be male or female, but i am assuming there was a long gown, to make him think it was a nun.

from 'the field guide to little people':
>"the White Women of Northern Europe fulfill many of the same functions as the local fertility elves of southern Europe ...
like the korrigans and the fees, they have been associated with the old pagan priestesses ...
>"the white women are helpful and generous towards humans ...help travelers find the right road, change flowers and stones into precious amulets, aid women in childbirth, make cows produce more milk, foretell the future, lessen the fury of storms ...
>"despite their generous natures, they can be easily angered by cruel, or ungrateful men ...
>"often seen near sacred groves ..."

• the boldface is my own emphasis, also i'm using an older edition than the one cited here on amazon.
  
  
• nuns and convents are the remnants of an older priestess order. this would be particularly true in ireland.
  
  
• an orchard is a kind of a sacred grove, though like a roman catholic convent, much diluted. apples in particular are sacred in celtic belief.
  
  
• my guess is that your father and his chums, through no fault of their own, acted in a way that overrode the sacredness of the space, and did not exhibit the kind of gratefulness for the gift of the apples that the white lady would have liked to have seen.
  
  
• when you (or anyone) are not appreciated or acknowledged, it is like you don't exist and you fade a little. perhaps what seems like an over reaction on the part of the white lady is in fact because she is in a fight for her continued existence in these modern times.
  
  if you haven't already, you might very much enjoy 'The Fairy Faith in Celtic Countries: The Classic Study of Leprechauns, Pixies, and Other Fairy Spirits' by wy evans-wentz the version i just highlighted has it for free on kindle. it is well well well worth the effort of reading. very long. can't remember if it specifically discusses white ladies. i doubt it. but it will discuss the tuatha de danaan, the great race of original irish beings from whom all true fairies are descended. the tuatha are shining white beings of great size, great age, and great power and are felt by many in celtic countries to be angels trapped for a variety of reasons on earth.
  
  edited to add: just noticed you said your town was noted for stories of bean sidhes. 'bean sidhe' just means 'woman fairy' and have the three aspects of 'maiden mother crone'. again, they all wear white. and they are believed to be the closest to the original tuatha de danaan