St.Mary's Clonmel is not a place it's a parish, for the sake of your family it is a Roman Catholic parish which includes among other places Clonmel town.
Your family lived somewhere in the parish, can you give us a date and name for some or all of the children so we can look at the record in the parish registry, sometimes we can pick up hints you might miss as we look at them so much.
Even better a link to the page on the NLI site
The St.Mary's in the Census is the Civil Parish