This family is quite confusing - especially with the re-use of names ...
Is Thomas, 18, in 1861 listed as a son of Thomas snr?
If so, it seems that we have:
*Thomas b Ireland ca 1843 (from the 1861 - presumably a son of an earlier marriage of Thomas snr? Relationship on 1861 census?)
*David ca 1852 (9 in 1861, birthplace?, relationship to Thomas snr? - presumably he did not survive as another son was named David later. This first David could be a son of an earlier marriage, or an unrecorded son of Thomas & Fanny, or a pre-nuptial son of Fanny's).
Then children of Thomas & Fanny (all except Armstrong listed in the IGI):
*Armstrong ca 1855 (6 in 1861); given that he is not in the IGI, perhaps he was b 1854 i.e. before statutory registration
*Samuel 1856 (seems likely to be chap recorded as Armstrong, age 6 in 1861), died 1856
*James 1857 (3 in 1861 - presumably he did not survive as another child was named James later).
*Henry 1860 (not in the 1861 - presumably he did not survive).
*Agnes 1863
*Fanny 1865
*David 1867
*Margaret Jane 1870
*James 1872
Has anyone searched for Armstrong in the 1891/1901 censuses under the given name of Samuel.
Did Armstrong (or Samuel) MCAULEY & Agnes FERGUSON have children?
JAP
PS: A free search on SP finds that an Armstrong MACAULAY, age 1, died in 1924. A descendant?
Note: Several later edits done so as not to mislead people with suggestions which have later been disproved.
PPS: There seem to be a couple of possible Samuel deaths:
Samuel MCAULEY, d 1899. He was 42 and died in Calton, Glasgow.
Samuel MCAULAY, d 1924 (mentioned earlier). He was 69 and died in Paisley, Renfrew.