He joined the army in 1879 at Portsmouth and gave his place of birth as Westport, County Mayo and his next of kin as father, John in Manchester, Lancashire. There is a reference below with is unclear but i think it says Brothers, none.
I've found him in 1881, training at shoebury, but his age here would give a date of birth around 1852. Place of birth Ireland.
No census for 1891 as he'd have been posted to Malta then.
In 1901, Thomas and family were living in Plumstead, birth here is 1857, Ireland.
They were still living in Plumstead in 1911 but here he gives his place of birth as Manchester, Lancashire. Date around 1856.
He again gives his place of birth as Manchester in the 1921 census, born in 1855.
I think you are making an assumption that it is always HE who fills in the census and other forms - even if he is head of the household.
And at that time people didn't always know their exact date of birth so fluctuations in years isn't uncommon.
He joined the army in 1879 at Portsmouth and gave his place of birth as Westport, County Mayo - it will be him providing the information to join the army so Westport as his pob looks good, although it may be the first place he can remember living rather than his exact pob
I've found him in 1881, training at shoebury, but his age here would give a date of birth around 1852. Place of birth Ireland. - again we have Ireland as pob and I'd expect it to be him providing the information
In 1901, Thomas and family were living in Plumstead, birth here is 1857, Ireland. I think he gave the info for this census
They were still living in Plumstead in 1911 but here he gives his place of birth as Manchester, Lancashire. Date around 1856. Does he sign the form? Does the writing look the same as whoever filled in the form? Maybe his wife or one of the children filled in the form and he signed it without checking it - maybe his reading skills weren't very good but he could sign his name.
The fact that his pob is now Manchester makes me think it is his wife or one of the children who has put down his pob as Manchester, a place they know or had been told his father lived and it wasn't HIM providing the information.
He again gives his place of birth as Manchester in the 1921 census, born in 1855. Again, does it look like HE filled in the form or might it have been another family member?
He died 1929 in Chelsea. Who is the informant? What age have they provided for him? Information on a death certificate is only as good as the knowledge of the person providing it.
Thomas and Susan's marriage cert gives fathers details as John McDermott, retired Mariner. So in 1886 he is claiming his father is John but not deceased? Are there any John McDermotts in Manchester in 1851/1861/1871/1881 who might be this father? Have you found a possible Thomas on the 1861 or 1871 census yet?
Just some things for you to revisit and think about. Hope it helps.