In 1967, Rosemary Sutcliff published the children's novel The High Deeds of Finn MacCool, which is a retelling of the stories of the Irish hunter-warrior Fionn mac Cumhaill and the Fenian Cycle. The stories around Fionn mac Cumhaill or Finn McCool have also been used in one of the most famous literary forgeries in English literature: the Ossian poems published by James Macpherson in the 1760s. These Ossian poems quickly became very popular; for example, Werther reads from them in Goethe's The Sorrows of Young Werther (first published in 1774). But Samuel Johnson already disputed their authenticity in his final works.
The fact that these publications were exposed as forgeries does not imply that they can't be used as a source of inspiration. So is there any evidence that Rosemary Sutcliff used James Macpherson's publications as a source?