Fiona Williams Tadhg is in the movie The Secret of Roan Inish, but was not originally a character in the book. The movie was considerably different from the book, including moving the setting from the original Scotland to Ireland, and changing a number of names. I am quite sure a lot of people were confused when they bought the movie tie in paperback, as, although it does contain stills from the movie, the story itself is the original one published in the 1957 in the U.K. and 1959 in the U.S.. I own both a movie tie in edition, and a British first edition hardcover. I have seen the movie a couple of times, but admit to preferring the original story, which I'd first read decades before the movie came out.
Nan Stephens Tadhg is known as a dark one because he is believed to have some selkie blood in him. Well more than the others. It all goes back to the idea that the shared ancestress of Tadhg and Fiona was a selkie woman Nuala. Tadhg shows more of Nuala's characteristics so he is referred to as a dark one.