There is little documentation available for Ireland for the 18c.
The baptism records for Kilkenny are available on line,www.rootsireland.ie
however, none this early. There is only one Church of Ireland Parish that has records back to 1728. Close but not to say that is where your John was baptised
Registry of Deeds for Ireland have records that go back quite early - lots of Green's but only
1 James Green 25 May 1798 Kilkenny
There are 3 Green's listed in the on line telephone book. Might be worth a letter or two? However, unless they are into genealogy themselves unlikely they even know about anyone who went to America. Usually these families are surprised when people turn up on the door step. You might be able to find someone with a common interest who is willing to visit some graveyards that still have readable gravestones, but first of all you need to know where in Kilkenny they were from.