It is based on the play Blackbird by David Harrower.
The nipple ring on display in the shower scene is apparently Rooney Mara's own, having had it done for the film ' The Girl With the Dragon Tattoo'
Filmed in new Romney, Northampton, greatstone and Littlestone in Kent.
Una is an old Irish name for a female. It possibly derives from the Irish uan (lamb). It is generally considered to mean "the personification of truth, beauty, and unity." Oona is a more common alternative spelling of the word.
Although from distinctly different sources, this movie is extremely similar to "The Girl in the Book," which came out the previous year.