... There is a Margaret Fitzsimons at that address on Griffiths Valuations
who appears to have died 1881 at Togher A daughter Kate was informant
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1881/06439/4850080.pdf
I wondered if Kate was the daughter
-in-law rather than just daughter as stated, ie. Michael Fitzsimon's wife Catherine Maloney nee Carroll. The only other baptism I could see for Martin Fitzsimon and Margaret Dunn was the following one in Raheen RC parish on 25th March 1846, 2nd entry down on the right hand page:
https://registers.nli.ie/registers/vtls000634556#page/58/mode/1up'25th Joseph of Martin Fitzsimon & Margaret Dunn of Clon---- ...' (I not sure that it says Clonard, maybe nearby Clonbane? Again I couldn't really make out the sponsors, maybe something like Patt Burk & Kitty Rorke)
While I couldn't readily see a marriage for Martin Fitzsimon and Margaret Dunn, a possible scenario could be that Martin was 41 when he was discharged after 2 decades of military service in December 1843, whereupon he married Margaret Dunn, say in 1844-45, and they had two sons Joseph in 1846 and Michael in 1849 (at which time Margaret was in her mid 40s, if her reported age at death is accurate).
The only military document I can see regarding Martin Fitzsimon, relates to a Board convened at discharge to determine his service, it corroborates that he was born in the parish of Rearymore
'in or near the town of Clonaslee', he originally attested for the 29th Regiment of Foot at Mullingar on 25 September 1822 at the age of 20, his reckonable service at discharge was just over 21 years, during which he served 11 years 9 months on the Isle of Mauritius, the remainder served at home, he was discharged as unfit in December 1843 (a chronic chest problem), and he was described as a labourer, 5'10.5" tall, with brown hair, grey eyes, and fresh complexion.