Interesting to know about the 11th Foot/Marines, I don't know much about military history.
I hope you don't mind me summarising but I'm starting to get confused
I agree that the two Thomas BURRELLs from Galway are the same one as the records seem to show a continuation/re-enlistment.
The first one as previously posted, that shows he enlisted in 1811 at Plymouth aged 24 then finished his service on 09 Jan 1821.
The second one shows he enlisted in Sheerness on the 10 Jan 1821 aged 33, so that would be right.
They also both give his occupation as Cordwainer.
The Chelsea Pensioner record also seems to be him as the April 1821 date and the reason of being there, chronic infection of the lungs, matches the first reason for discharge.
There does seem to be a small discrepancy in the age of the Thomas BURRELL above who says he is born abt. 1787 and the age of the Thomas with Elizabeth in 1851, born abt. 1784 but this often happens so doesn't mean they are different people
I can see the baptism of Thomas jnr in 1824 when Thomas Snr is called a Royal Marine, would he still be called that if he had been discharged?
Have you found the marriage of Thomas and Elizabeth?
What happens to Thomas jnr after that?
Sorry, so many questions but it is a very interesting puzzle
Daisy