What is the title of an Earl's daughter?

On the current PBS BBC show "Downton Abbey" they call the Earl's daughters Lady Sybil and Lady Mary but in the movie "The Importance of Being Earnest" they called Lord Bragnall's daughter the Honourable Miss Fairfax, so is an Earl's daughter really a 'Lady' or just an 'Honourable Miss'? Does it depend on whether there is "female remainder" or was Lord Bragnall just a baron?