Sarah-Jane,
In general, people in Ireland in the 1800s didn’t celebrate birthdays, didn’t have birth certificates or passports (though they might sometimes have had a baptismal cert) and often had little accurate idea of their ages. Most ages on official documents were just a guess. An error of up to 10 years on a death certificate for someone in their 70s or 80s was common.
I don’t think it’s likely that there were 2 people both named Leslie Mitchell, of around the same age, both tailors, in a small village like Kilrea.
I have attached a letter which I found in parish records in PRONI (the public record office) in Belfast from someone in Pettigoe, Co. Donegal in 1908 writing to his Minister, asking for proof of age (ie a baptismal certificate). All he knew was that he was between “70 and 78 years of age.” He clearly had only the vaguest idea and couldn’t narrow it down to within 9 years. (The likely reason for the letter was that the old age pension was being introduced in 1909 for people aged 70 and over. Documentary proof of age was required. Thus, probably for the first time in his life, establishing his age accurately became relevant to him).
You have Eliza Mitchell who died in 1915 as your great great grandmother. I don’t think that’s correct. That Elizabeth was married to James Mitchell (who died 1913). James was Sarah’s brother. Here’s the 1915 death:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1915/05284/4468058.pdf
Here’s James death:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_retu…
The 1883 death certificate for Leslie Mitchell says he was married, so his wife was still alive then. I searched for her death 1883 onwards but did not find it. She’s not in the 1901 census, so I assume she was dead by that year or had left the area.
One troubling aspect is Sarah Mitchell’s marriage (31st December 1877). She gives her parents as Leslie Mitchell and Elizabeth McCartney. You would expect her mother’s maiden name to be Beresford. She also indicated both were still alive in 1877. Sarah married after banns according to the Episcopal church so that fits with being Church of Ireland. (The 2 witnesses were William & Isabella Mitchell. Do you know where they fit in?).