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.
Through Ireland Births and Baptisms, I found:
Joseph Young, born 4th Jul 1867 (so I assume the earlier Joseph died young and they named the next boy Joseph)
(Unknown) Young, born 3rd Jan 1869
(Unknown) Young, born 4th Nov, 1870
Sarah Louise Young, born 6th Sep, 1872
Edith Young, born 6th Sep, 1872 (twin to Sarah)
Jane Young, born 21st Jul, 1874
The first three show birthplaces of Coalisland, and the latter three show Belfast, so it appears the family moved around 1870-1872.
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".
Do you have independent confirmation that these are the same Elizas? It seems pretty clear, since the maiden name is Jackson. I examined both marriage records, and Eliza's signature looks roughly similar in each, but she uses "J." in one and "Jane" in the other. It's hard to read, but it appears the marriage to Robert Stewart was witnessed by a young Florence Young, which would seem to provide a second confirmation of the connection.
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.
There are death records for a Sarah Louise Young and Edith Young, both in Belfast, 1872, 0 years old, so these may be Joseph and Eliza's daughters. There is a death record in New York City Municipal Deaths for a Joseph Young, son of Eliza Jane Jackson and Joseph Young, born Ireland, buried in Green-Wood Cemetery on 27th June, 1906, aged 35. (This is also where many of my Jackson ancestors and Ruth Galey are buried). There is also a death record for Jane Young in 1880, estimated birth year of 1774, but there are no parents listed in the extract, so I am less certain of this identification.
I had traced Ruth through census records but did not know about the will. That's a great tip! I also traced Margaret through the US census -- she married George McCarroll and had three children: Jane, Thomas, and George Jr. As far as I can tell, all three died without issue.
Here's a forum where someone was asking about Sarah Louisa Jackson, daughter of William Jackson from Carland Presbyterian Church:
https://www.ancestry.com/boards/localities.britisles.ireland.tyr.general/3083.1.1/mb.ashxI compared her marriage record to Eliza Jane's, since both hold William's signature. Again, they look somewhat similar, but one says "Wm." while the other writes out "William". Eliza's is not scanned well, or in bad condition, so it's hard to make out. But the way William writes "Jackson" in each looks similar, with an oversized k and the following three letters looking more like four. I would be willing to bet they are signed by the same man, but only a small amount.