...whereas Dalrymple is the title ;-) They were often not the same.

As a widow of a Viscount ,Sir Walters cousin's title was the Dowager Viscountess Dalrymple. She had been married to the Viscount Dalrymple. His family surname was Carteret, thus his daughter, whose honorific title is the "Honourable", takes her father's surname for she is not entitled to be called "Dalrymple".

Here is a link to Henry's Jane Info pages about titles which may help you further, Marianne.