Rob,
A little bit of background to explain the 1851 census information you have been using. The state pension was introduced in the UK and Ireland in 1909. To qualify you had to be 70 or over. You had to prove your age. Birth certificates were only introduced in Ireland in 1864 (as opposed to 1837 in England & Wales and 1855 in Scotland) so in the early years people born there wouldn’t have one. A baptism certificate was acceptable instead, as were military discharge papers and similar documents. For those without any such documentary proof, an option was to check the 1851 (and sometimes the 1841) census, to see what your age was then. If that made you 70 or over, then it was acceptable as proof of age.
So Robert appears to have applied for a pension when he reached 70 years of age around 1921, and he evidently went for the 1851 census option. Now we know he was aged 2 in 1851, so in 1920 he probably couldn’t recall exactly which townland his family were living in. In such cases, people stuck down the names of any townland they thought the family may have been in. (I have seen that many times before). In your case that was Boyher, Drummulla, Monneill & Rockcorry. The person who searched the records would then search each townland given on the form. In this case he evidently found the family but it isn’t clear to me from looking at the document on the NLI site which townland they were actually living in. Though since Mary Anne White was in Boyher 50 years later, perhaps that was it.
Mary Ann White in the 1901 census was Church of Ireland (ie Episcopalian). You could search the Ematris baptism records to see if you can find Robert and his brother Henry c 1849. That might give you a little extra information, including the father’s name. The baptism records for that parish start in 1811. There’s a free copy in PRONI (the public record office) in Belfast. Or you could contact the church (expect to pay a fee if the Vicar agrees to do the search).
I note that John Hendrie is reported to have been a policeman. If you think he served in Ireland, then it was probably in the Royal Irish Constabulary. You may be able to trace his service record. There’s copies of the RIC records in various location eg NAS, Kew London, NAS Dublin, and PRONI in Belfast but if you can’t access them, then the police museum in Belfast will do a look up for you for a £25 fee. https://www.psni.police.uk/inside-psni/our-history/genealogy/
It was RIC policy not to post a policeman to a county where he was born or had family connections in. So if he served in Monaghan, he wasn’t born there. The service record will normally tell you the county he was born in though.
Having said that, Robert’s marriage certificate he gave his father’s occupation as bookbinder. The information on the marriage certificate is likely to be more reliable than the death certificate. The marriage certificate information came from Robert who was more likely to know his father’s occupation than the informant on the death certificate who almost certainly had never met Robert’s father and probably knew very little about him. Information on death certificates is often the informant’s best guess and consequently mistakes are common.
The 1904 death you have spotted for Mary White isn’t the right one. That person was a farmer’s widow lived in Greagh. This is her in the 1901 census:
http://www.census.nationalarchives.ie/pages/1901/Monaghan/Kilmore/Greagh/1637615/
Boyher falls within Cootehill registration area. I searched Cootehill deaths. I think your Mary Anne died 26.1.1912 in Rockcorry. She was single, a labourer, aged 80, and cause of death was senile atrophy. Informant was Mary McBennet, who was present at the death. This looks to be Mary Anne in the 1911 census:
http://www.census.nationalarchives.ie/pages/1911/Monaghan/Dawsongrove/R…
