Hi
Does anybody know where this church in Wexford is ?
It is located in the townland of Barntown. That is why it is Barntown church….

This is not simply tautology, there are many sources to locate Irish townlands, online maps, etc.
As for why you have not located parish records for Barntown, that is also straightforward - Barntown is not a parish, it is part of the parish of Glynn, i.e you need to look for Glynn records.