There's a possible death, registered in Dungannon district, in the name of Michael Magill of Lisnamonaghan who died on 20 March 1881, he was recorded as a 47 year old married
army pensioner, the informant present at death was Ann Magill of same address. Also possible that the informant was this person in Lismonaghan in the 1901 Ireland census, note she was a widow and a
seamstress:
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Castlecaulfield/Lisnamonaghan/1738475/Note also that a widower called Michael McGill of Kinego (occupation not known) married an Ann Duff of Lismonaghan (a
dressmaker) in the RC parish of Donaghmore on 18 June 1876, Michael's father was also called Michael McGill, a farmer, Ann's father was James Duff, a farmer, the witnesses were Thomas Quin and Mary Ann Kelly. So possibly married again after leaving the army?
On a separate tack note that a Michael McGill and Margaret Camley / McCambley married in Dungannon RC parish in 1826 and had a child baptised in same parish in 1828, they were living in Killymaddy at the time of the baptism, which isn't far removed from the geography so far, nothing yet to say these were Michael's parents, just to note until able to rule in or out.
Dungannon RC parish registers are here:
https://registers.nli.ie/parishes/0214