Many thanks to Mr James Nyhan, a colleague of my husband, who has sent me a link to an article by a Slovenian folklorist, Monika Kropej: 'The Tenth Child In Folk Tradition' which casts light on who or what the Djachwi is, in the Irish tale of my last post, 'The King Who Had Twelve Sons'.
Do read it - it's extremely interesting, and it seems that the word stems from the Old Irish word for ten or tenth, and refers to the legend that the tenth child (or in some cases the seventh or twelfth) must roam the world as either a sacrifice to (a tithe) or as a personification of Fate or Destiny.
Kropej even quotes William Larminie's missing note, torn out from the back of my book! But the article is much more wide-ranging than that. (There's a wonderful tale of the tenth daughter, the only one of her sisters to respond to the Virgin Mary walking through the fields, who therefore has to go away with her. And who says to her own mother, "You shall not see my death, but I will be standing by you when you die.")