On 18 Nov 1880, according to FamilySearch, "Letters of Administration of
the personal estate of Rupert John Mackintosh late of Clonderrig Queen's
County Gentleman deceased who died 19 March 1875 at same place were granted
at the Principal Registry to Elizabeth Mackintosh of said place the Widow
of said deceased."
While it's possible that there were two different Elizabeth Mackintoshes, I
doubt that many wives in the Victorian era would have retained their maiden
name upon marriage.