Well that would make John and Margaret born same year.
"Mother Jane Carson married 1844 Monaghan ". but does it give anything else? Born where?
It's difficult to get a complete picture without the complete data. while you've set it out well, concise etc having to ask question after question usually ends up in a thread that gets nowhere as the flow of posts makes it impossible to follow.
So as far as I make it out it is;
"Researching my great grandmother, she is born in 1855 in Kilbirnie Ayrshire.
Father Thomas Carson occupation ironminer born 1826 County Armagh.
Mother Margaret Hall born 1825 County Monaghan. Died 1884.
They are married in Kilbirnie in 1854 .
That is all I have on my great grandmother Martha's parents.
The 1855 birth certificate from Scotlands people has a birth on the bottom.
James Hall born 1855, Kilbirnie ayrshire .
Father John Hall, age 30, born County Monaghan.
Mother Jane Carson, age 28, married 1844 Monaghan . Possibly 23rd Dec 1845 John Hall son, Henry Hall, Tirnaneill married Jane Carson dau, Philip Carson, Legacurry at Donagh, Co Monaghan.
If you have anything to narrow it down further it will help, you have 2 Halls, 2 Carsons with 2 Kilbirnie births so did these Hall/Carsons attend same church, any census records for them, did they live near each other etc in Scotland? If they were brother/sister, any sign of Henry there? Are there burials for them, if so, same cemetery?
Thomas Carson son of who?? (Philip???). His death cert should give this. Then you'd know if he was possibly Janes brother or not.