I don’t know much about British Army records but my understanding is that the main collection is in the National Archives, Kew, London, and that though some are on-line eg medal records and obviously the pension records you have found, many are not. So you usually need to go to Kew (or get a London based researcher to go) to search for additional records. You know what unit he served with and I assume therefore it shouldn’t be too hard to track them down.
I can see that the pension was being paid between 1867 and 4th May 1890, so presumably William died around 1890, somewhere in the Moira area, and so should be registered in Lurgan. These are the only 3 Wm Hendron deaths in the Lurgan registration area between 1867 and 1899:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1872/020751/7280064.pdf
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1886/06244/4787735.pdf
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1883/06348/4821053.pdf
None seems a particularly good fit. Ages don’t fit very well and none is described as an army pensioner, though that wasn’t mandatory but is often what you find.
