OK. There's nothing of any significance marked on the maps. There's a holy well and a graveyard a little over midway between Sallins and Newbridge, but they're on the other side of a road so it's not either of them. You say 2 minutes out of Sallins heading towards Newbridge – how long does the train take to get to Newbridge from Sallins so I can get a better bearing on the place?
There's a church marked on the map north of the train line just after it crosses the Liffey out of Sallins to Newbridge (to the right of the line). Quite often all that remains of these sites are very low walls covered in vegetation. Might this be it? Are you sure it's on the left as you leave Sallins?
Andy