Water Pumps, a Mystery Arch and New Abbey

Image courtesy OSI (Tailte Éireann)

I met John Brady in New Abbey Cemetery this evening and amongst other things, we were talking about hydraulic rams or ram pumps, According to John, there was one located near one of the three small bridges over the Mill Stream, the newest of the bridges carrying the mass path. The pump I think was for pumping water to New Abbey house. I knew there was also a hydraulic ram located on the Gilltown estate, north of the lake, as it's visible on the circa 1900 25" OSI map.
I still haven't worked out what the small arch was for, the top of which used to just protrude above the water line of the stream. This was located in a wall adjoining the stream, between the mass path bridge and one of the two older ones further south. It was visible when we were kids, looking for the legendary, but highly unlikely tunnel connecting New Abbey to the town or Old Kilcullen. We were able to push long sticks into the archway, so the cavity went in some distance under the wall. The area at the base of the wall is now however choked with weeds, so the arch is no longer visible. Possibly it was a drain from the house that used to be just inside the wall and arch at the end of the cemetery car park, (visible in the the Lawrence Collection photo of New Abbey I linked to in the comments). There were also other buildings in the area, visible on the first edition 1837 OSI map. The field occasionally floods (the last time in 2015 or 2009 from what I recall) and another possibility is that this could have been a drain to allow flood water to flow back into the stream. It could also have been the water intake for a channel that flushed waste in a crude latrine system, something I've seen at the ruins of the woollen mill in Ballymore. Maybe a question or a piece of information to add to Michael O'Connell's jigsaw. Anyway this took me down a Wikipedia rabbit hole (a regular occurrence!) to read about self-powered water pumps which were often used for raising water to a higher level before electricity was available.