A Nathaniel Speer, who may or may not be Sarah's father, died 10 Dec.1864 aged 42 years. Death certificate not yet on Irish Genealogy site (will be available from GRONI, as the 1868 marriage, after registering and purchasing a few credits).
Newspaper death notice- (extract) in Derry Journal, 17 Dec.1864-
December 10, in this city, Mr. Nathaniel Speer, gardener of Gwyu’s [?] Institution, deeply regretted.