from a fascinating article with comments by both Friel and the author of the book Paper Landscape (John Andrews); Andrews's book partly inspired the writing of Friel's Translations.
Writing a historical play may bestow certain advantages but it also imposes particular responsibilities. The apparent advantages are the established historical facts or at least the received historical ideas in which the work is rooted and which gives it its apparent familiarity and accessibility. The concomitant responsibility is to acknowledge those facts or ideas but not to defer to them. Drama is first a fiction, with the authority of fiction. You don't go to Macbeth for history. (p. 124)
(Barry, Kevin, Brian Friel, and John Andrews, "Translations and a Paper Landscape: Between Fiction and History." The Crane Bag 7.2 (1983): 118-124.)