I know the servants still referred to her as Lady Sybil, but would new acquaintances and/or people who weren't on a first-name basis with her have called her Mrs Branson?
Before marriage, the daughters of dukes, marquesses, and earls are entitled to be called "Lady [First Name]", while the daughters of viscounts and barons are addressed as "Miss [Last Name], but may be referred to in writing as "the Honourable [First Name] [Last Name]". Thus, the daughters of Lord Grantham, who is an earl, are addressed as "Lady Mary" or "Lady Edith", but the daughter of Lord Aysgarth, who is only a baron, is addressed as "Miss Allsopp" (although you would address the envelope of a letter to her with "The Hon. Madeleine Allsopp".)
After marriage, the way the daughters of dukes, marquesses, and earls are addressed depends on whether or not they marry a peer. If the woman in question marries a peer --and regardless of the peer's rank -- she assumes his style. Thus, if she married Visciount Gillingham, Lady Mary Crawley would stop being "Lady Mary" and would instead be "Lady Gillingham." On the other hand, if she marries anyone who is not a peer she continues to use "Lady" and her first name, combined with her new last name. Thus, when Lady Rosamund Crawley married Mr. Painswick, she became Lady Rosamund Painswick, and was addressed to her face not as Mrs. Painswick, but as Lady Rosamund. Note that this rule applies even if the woman marries a baronet or a knight. If, for example, the scheming Edna Braithwaite had snared Sir Anthony Strallan (and yes, that is silly, but we are using the example of a commoner's daughter marrying a baronet or knight), she would be addressed as "Lady Strallan", but if Edith had married Sir Anthony, she would still be addressed as "Lady Edith."
In the case of Sybil, she would properly be entitled to be called "Lady Sybil Branson", and to be addressed by her neighbors and co-workers in Dublin as "Lady Sybil". Naturally, in those circumstances, that form of address would seem a little bit odd to anyone hearing it (because who expects a working nurse to be the daughter of an earl?), and no doubt when people in Dublin first met Tom Branson and his wife Sybil, they naturally assumed they were simply "Mr. and Mrs. Branson", and called them that. Sybil mentions this to Mary -- that in the lower class social circles in Ireland where she now travels, she is usually called Mrs. Branson, and Mary recommends that Sybil not tell this to Lord Grantham, who would find it offensive that his daughter was not receiving her proper form of address.
In simple terms Between 1985 and 1919, Sybil was known as "Lady Sybil Crawley"; after marriage, she became "Lady Sybil Branson" but everyone - since she was born - would have just referred to her as "Lady Sybil". Those in Ireland would have called her "Mrs Branson".
Edith is known as just "Lady Edith" or "Lady Edith Crawley" in official places (the season in London, etc).
Mary is known as "Lady Mary" or "Lady Mary Crawley". With Mary, the interesting thing is that her marriage to Matthew - as he has the same surname - does not change her title. Yes, she could be known as "Mrs Matthew Crawley" (for Isobel was known as Mrs Reginald Crawley in Series 1 Episode 2) or "Mrs Crawley" but these probably won't be used to differentiate between Mary and Isobel.
What do you think?