I found the 'forgotten' descriptor in the title something of a misnomer, given that I recognised most of the original tales from the folklore anthologies I used to read as a teenager

that's how I felt about it too! There was one story where the original was even exactly the same as some stories we have in Icelandic folklore (possibly through cultural exchange in the time of the Vikings since a good number of the settlers were celtic) and a few others were very familiar as well. But I also read volumes upon volume of (mainly Icelandic) folklore collections when I was a child...