Ok Naas is about 6 mins drive (i guess) from Punchestown. Dunno if this link works
Its a google map link.
The only thing is that road is a main road, so I am guessing they wont want people walking on it,then again there are gonna be so many of us I dont think they will be thinking of an alternative.
Have made a few enquires about that road and its walkable. Am staying in Naas too. Also tried Bus Eireann to see if they still have tickets left to get back to Dublin.