Mrs. FitzGibbons is to Castle Leoch what Madam Pomfrey is to Hogwarts. Well, if Madam Pomfrey also did all the cooking and cleaning… without magic. Mrs. FitzGibbons is basically in charge of everything, and she's a great help in welcoming Claire to Castle Leoch.
Claire remarks at one point that Mrs. FitzGibbons would be an excellent nurse, and to this end, Mrs. FitzGibbons helps Claire with whatever she needs—boiling water, clean bandages—without asking questions about Claire's modern methods, unlike many people who regard her ways as magical and sinister. Claire even learns a thing or two about ancient medicine from Mrs. Fitz—namely that leeches can actually be useful.