Dunn, who directed McLynn in Gypo and in the recent shoot for The Calling, is working with McLynn on the untitled comedy drama.
McLynn, best known for her TV role on Father Ted, is also a novelist.
The new film will be part of Dunn and producer Elaine Wickham's three-picture deal with Target Entertainment. Now in post, The Calling, about a university student who decides to become a Benedictine nun, is also part of the deal. The latter stars Brenda Blethyn.