I have added a thought to my post about stonework at Clonsilla. To save readers from having to open that page, here is the text.
Peter Clarke, in The Royal Canal: the complete story Elo Publications, Dublin 1992, points out that, in 1807, there was a passenger service from Dublin to Clonsilla: the six miles cost 1/7½ in first and 1/1 in second class.
Could it be that the passenger station was under the bridge, with access controlled by gates at either end? Horses could have been changed too, with the ramp providing access for horses to the road. Passengers too could use the ramps, but horses could not use steps. And, as modern canal users will attest, it is always easier to embark and disembark passengers under bridges, where there is deep water at the edge and where the boat does not have to go off its course.
If that is so, there might be similar stonework at the other passenger stations that were located at bridges rather than at harbours. There would be traces of gate pillars at either side of a bridge. Ramps would be required only where the canal bank’s level was significantly above or below that of the road.
Irish waterways history 