Hi Faith,
There is a gravestone in St. Nicholas' Graveyard in Carrickfergus that bears the inscription:
In loving memory of Robert Johnstone who died 21st January1869 aged 82 years. And his wife Mary, who died 28th August 1868 aged 76 years. Erected by their sons R. and C. Johnstone. Also Margaret, wife of R. Johnstone, who died 1st January 1905 aged 86 years. Robert Johnstone who died 11th April 1916 aged 96 years.
Robert Johnstone of Silverstream, Carrickfergus, a farmer, who died 19 Jan 1869 had his will proved at Belfast on 5 Feb 1869 by Charles Johnstone of Silverstream.
This could very well be the 'Johnston' family your gg grandmother referred to.
If you think there is a connection send me your e-mail address by PM & I'll pass it along to a Johnstone researcher in the U.S. that has Johnstone ancestors from Silverstream & is most likely linked to the family of the above gravestone inscription.
Good hunting.
J.A.M.