The Kilcock park-and-ride thing has actually been looked at in the past. I remember the 2000 Scott Wilson report on Dublin Bus (and Bus Éireann within the Greater Dublin Area) mentioned it in passing:
http://www.dublinbus.ie/about_us/pdf...on.pdf#page=73
I think the DTO's favoured option, however, was a park-and-ride near Intel in Leixlip, with traffic coming off the M4 at the Celbridge interchange. Since then, Kildare County Council chose the Leixlip site as the location for a new town centre to serve the Leixlip-Celbridge-Maynooth triangle (over the advice of their consultants, who favoured expanding Maynooth town centre). This would seem to rule that location out, assuming the town centre proposal is still alive.