The actress, who has previously appeared in Father Ted and Jam & Jerusalem, will first appear in series seven when it returns to Channel 4 later this year.
McLynn will play a local Librarian called Libby, who becomes a new love interest for Frank Gallagher. Valerie Lilley is also joining the show as Libby's mum, Patty.
"We are thrilled that the acting talents of Pauline McLynn and Valerie Lilley are to be joining our already celebrated cast on the Chatsworth Estate," said producer Lawrence Till.
"This season we will continue to push the boundaries of storytelling: keeping the audience on the edge from the first frame to the last blast."