I am so pleased to find my Gr Grandad burial place I am going to visit Birr in a few weeks and
now its made it worthwhile ,thank you so much
His Name was Colour Seargent . John Marks of the 66th foot reg
I'm sure you've figured it out already, but the sergeant buried in Crinkill is unlikely to be your great grandad as the soldier buried in Crinkill was in the 3rd Leinster Regiment and had no service with the 66th.
The list on FindAGrave is fairly incomplete, the full list of those buried there with and without a gravestone should be published sometime next year.
Hope this helps.