Hi there,
I'm Noreen, the parish liaison for the neighbouring parish of Ballingarry and I have family links to Killenaule.
Having a photo of the family gravestone is a great clue - you can see the names and dates of death which can help you confirm if its the correct family.
Bear in mind that ages noted on gravestones may be estimates depending on who informed the priest or stonemason.
Have you checked the free Irish civil records site for the names and dates of death? Link below:
https://civilrecords.irishgenealogy.ie/churchrecords/civil-search.jsp
This site will give you access to the death records which you can download and save. I had a quick look and the grandson inscribed as what looks like 'Jermine' is actually James on the death records but was the son of a postman and his mother Bridget registered the death - which ties in with the information you already had. Note, on the 1901 Census a son named James was recorded aged 8 - often after the death of a child the next child of that gender was given that name especially if they were named after a grandparent.
I hope this has helped point you in the right direction.
Best of luck,
Noreen