Waterstown. From Sallins, if you head west along the Grand canal there's an aqueduct where the canal crosses over the Liffey. The Grand Canal Way is on the North side of the canal but you need to be on the South side (if there's a path). Just beyond the aqueduct, on the south side, there's a lane/track/boreen. Walk to the end of this and the church is about 200 metres due South.

Alternatively, head South out of Sallins towards Naas. Take the 1st right turn and then turn right at the next T junction. This road will take you back to the canal and you will spot the aqueduct and follow on as in above.

It seems like a nice little walk but all of this is deduced from the map (Sheet 49) so for all I know I could be sending you into the biggest bog in Kildare (oh no, that's the Bog of Allen :-))

Andy