I've been trying (unsuccessfully) to get any further with the problem of whether the husband of Mary SCULLION was Thomas FITZSIMMONS or Robert FITZSIMMONS.
For my own benefit, I'll recap the findings so far (lots of work by ibi!) and ask some questions - please don't hesitate to point out any misunderstandings.
1867
Thomas FITZSIMMONS marries Mary SCULLION. His age recorded as 22 (i.e. b ca 1845) and occupation as engine driver.
1868
Daughter Annie born. Registered by her SCULLION grandmother. Father's name given as Robert
1870
Son Thomas born. Registered by his father whose name is recorded (and signed? he made his mark - see reply #14 by ibi) as Thomas.
1871 census
Mary, Annie and baby Thomas are with Mary's parents.
As far as I am aware Robert/Thomas has not yet been found in this census or in any subsequent census?
1874
Mary dies. Husband's name given as Robert.
Who was the informant? Informant was Robert FITZSIMMONS who signed his name (see replies #11 & #14 by ibi)
ca 1877
Annie dies aged 9.
Who was the informant? What name is recorded as her father?
1897
Thomas jnr (b 1870) marries. His father's name is recorded as Thomas Robert (see reply #8 by scully).
Did Thomas jnr ever know his father? As a baby in 1871 he was with his mother and sister (but not his father) at his SCULLION grandparents. In 1881 and 1891 he was with his Uncle John O'HARA and Aunt Ann (FITZSIMMONS) O'HARA.
Date??
At Thomas jnr's death (informant?) and/or burial, his father is recorded as Robert, an engine driver.
Did Thomas/Robert ever, himself, give his name as Robert? Or only as Thomas? Yes, as informant of the death of his wife Mary in 1874 he gave his name as Robert - see above under 1874
There is a possible Robert in 1851 with appropriate parents - but age does not quite fit the marriage age (perhaps he lowered it a little because Mary was only 19).
1851
Ayrshire, Newton on Ayr, Saltfield St
FITZSIMONS
Thomas 53, Coal Shipper, b Ireland
Helen 44, wife, b Ireland
Mary 17, daughter, Woolen Factory, b S Newton, Ayr
Helen 15, daughter, Woolen Factory, b S Newton, Ayr
Daniel 12, son, Woolen Factory, b S Newton, Ayr
Robert 9 (i.e. b ca 1842), son, Woolen Factory, b S Newton, Ayr
Helen D HARA (sic) 1, granddaughter, b S Newton, Ayr
The only one I've traced is Daniel - through to 1901 (under some pretty innovative spellings as transcribed). He married a Catherine CURRIE in 1859, and was an Engine Keeper (or similar) in all censuses after 1851.
Was Mary listed as married in 1871?
Might Thomas/Robert have died or decamped after the birth of baby Thomas? No.
Who was the informant of Mary's death in 1874? Robert was. See above under 1874.
JAP
Note: edits/corrections are made in red above (many many thanks to trish251 for her very helpful pointers).
Note: Robert, with father Thomas & mother Helen, has now been found in the 1861 census (see later - reply #27 by ibi). Robert's age is given as 17 i.e. born ca 1844.