Thank you all for posting these helpful replies, specifically:-
Conahy calling, pet50ite, and eadaoin - and I shall try your suggestions
Heywood - thank you, yes. Although I am not sure this Michael is my ancestor (his full name was Joseph Edmondson Masterson, after his mother's surname) I have looked to see what happened to Mary Anne, thinking she may have died early, leaving Joseph as the only child which family tradition says he was. But I found no reference to her later, e.g. death or marriage.
If Artane opened only in 1870, the1854 Joseph was then 16, the age dathai says pupils left. Perhaps the Christian Brothers ran an earlier school on the site, or the 1854 Joseph was not my ancestor. Either way, seeing the original church record entry is my first objective.
dathai - I think the Artane you speak of was not what it was 60 years before when my ancestor was there, He was a US Navy volunteer seaman after leaving, and had an article about his voyages published later in the Artane school magazine, which I have. The other evidence that he was a pupil is a poem he published in a Glasgow newspaper when he lived there, which referred to the good education he had at Artane.
Again, thanks to you all, and to any others who can help.