Many graves in Ireland don't have headstones, all sort of reasons.
Even if there was one and it could now unreadable, try looking for The memorial inscriptions done 1900. I know from looking at Brian Cantwell's Memorial Inscriptions of 1985 for Wicklow and Wexford, there were many he could no longer find but that had been in the 1900 survey.
There is an English company that has the records of the 1900
Survey, it is a pay website and you have to have a full membership before you can even look. You can get the website by emailing me direct at firstname.lastname@example.org
as rootsweb/ancestry don't like commercial websites addresses shown on the message board.
Graigue Abbey or Graignamanagh was surved.
In the 1850 Griffith Valuation there are 4 John Doyle's in the Parish of Graignamanagh, none in the Townland of Coolroe.
People erect memorials in graveyards even today. It doesn't have to be on the unknown grave. I'm sure you you write
you can make arrangements for a memorial. I've also seen
people donate a plaque for inside the church in memoriam.
I know of people donating a bench with the memorial on a plaque where people can sit and contemplate. There are
a number of ways I'm sure that the clergy will happy to