Thank you Number Theorist and Aghadowey for your inputs.
I am rather overwhelmed by the information that the g grandparents had so many children! I thought it might help if I put down the birth dates of the children that I have seen in registers.
Eliza and Joseph Young had:
Robert Young born 25th June 1861
Florence Elizabeth Young born 11th January 1863 (My grandmother)
Joseph Young born 7th October 1864
Margaret Jane Young born 28th June 1866
I have not got a date of birth for Mary Aitken Young, but know that she was "46" in the 1910 USA census.
At some point Joseph Young must have died, because Eliza Jane nee Jackson married Robert Stewart Bachelor on 3rd April 1882 in May Street Presbyterian Church and she was described as "widow".
Eliza Jane had two daughters, who were described as "Stewart" in their marriage notices, but I am not sure if this is because they were still living with her and had been adopted unofficially by Robert Stewart, or if they were actually daughters of Eliza and Robert Stewart. They were both born, apparently in Co. Antrim, but I have not yet found details of their births.
These two daughters were:
Grace Josephine Holmes "Stewart"
Jeannie Caroline "Stewart"
Their marriages were documented thus:
11th October 1906 at Malone Presbyterian Church
Norman Wilson Bachelor, Solicitor, to Jeannie Caroline Stewart.
Both resident in Belfast.
(Jeannie Caroline "full age" therefore born 1885 or earlier.)
WITNESSES: Herbert Weiraker and GRACE JOSEPHINE HOLMES STEWART
12th April 1911 at Cregagh Presbyterian Church
Richard Henry Smith Bachelor Clerk and GRACE J. HOLMES Stewart.
Grace was full age and older than Jeannie Caroline, therefore born earlier than 1885.
The census information for the Stewart family in Belfast is very inaccurate, allowing Eliza Jane to age 15 years in the 10 year period between censuses and Grace scarcely to age at all between censuses. Jeannie Caroline had already married and left home by the second census.
My father, son of Florence Elizabeth Young, was very fond of his two "youngish" aunts and the entire family was very fond of Robert Stewart, even naming Florence Elizabeth's second son "Robert Stewart" after him.
I think that Eliza and Joseph's infant sons perhaps did not survive infancy. I wonder what happened to Edith, Sarah Louise and Jane. I also wonder what happened to Joseph Young, what date he died, whether he was the father of the two youngest girls or not. If I could find their birth registration, I might find out.
Mary Aitken Young/ Marley had a daughter, Ruth Jackson Marley who married John Galey, an American lawyer and they lived in Greenwich Village in New York. She had no children and I do not know what happened to her brother. Ruth Galey wrote regularly to her cousins in Belfast. She died in December 1966.
Mary Aitken Young/ Marley announced the death of another aunt: Eliza Young, 75, native of Dungannon, on January 17th 1907. The New York Herald added the note: "Toronto (Ontario Canada) papers please copy" Presumably some members of the Young family were in Canada. I still know nothing of Joseph Young or his family, but assume Eliza Young was his sister.
Carland Presbyterian Church is nearer Mulnagore than Dungannon and I did find some reference to William Jackson being connected to Carland, though which William I am not sure.