There is no photograph nor sketch of the nun. Was she the nun from Burres who was walled up in the 14th century? If so, she would probably have a different habit than Marie Lairre from France who was allegedly murdered in 1667. The descriptions from the original sources describe her this way:
". . . all three simultaneously saw a female figure, with bowed head (1900). She was dressed entirely in black, in the garb of a nun. She appeared to be telling her beads, as her hands were in front of her and appeared to be clasped. . . . The figure was slowly gliding - rather than walking. . . .She looked intensely sad and ill. . . .She had an expression of intense grief on her face. . . . It was solid, like a human being, not a subjective image, [or] a phantasm. . . ." - MHH, p. 44-45.
"The Coopers (1916-20) have frequently seen the nun, or at least a black shape resembling a hooded figure. . . .One bright summer's evening, about ten o'clock, they saw the figure. . . .dressed as a Sister of Mercy." - MHH, p. 54-55.
"Mr. Cartwright (1927) saw a Sister of Mercy. . . . she seemed pale, tired, and ill." - MHH, p. 56-57.
"Mrs. Lloyd Williams (1938). . . had distinctly seen a round dark object. This might, she said, have been a short stoopping figure." - MHH, p. 218-19.