I’m going to Co Roscommon tomorrow in the hope of being able to see what remains of the Rockville Navigations. I know the lakes are still there, and so are some of the connecting channels. On the early Ordnance Survey maps, only two of the channels are marked as canals; both of them are near Rockville House itself. There are also two subsidiary canals, one of which may have fed the fish ponds. Aerial photos show that at least one further stretch was canalised after the OS map was drawn, with a curved canal replacing a zigzag river.
What I’m most keen to see is the canal bridge that was near Rockville House itself (the house is now demolished). It may be the only stone structure that shows the width of the navigation (probably a lot less than the Grand and Royal Canals!), as there were no locks. I understand that the bridge still exists and that there is a date on it; that would be interesting because I haven’t yet found any information on the construction of the system.
If you can go to this page and line up the quarry (a white scar on the landscape) in the middle of your screen, then move down to the bottom, you’ll have two small lakes on the left and a bit of one on the right. You can see the channel connecting the rightmost lake with the middle one: it starts from the right as a curve, then becomes straight. About one third of the way along the straight section, a straight road or track crosses the channel, and I think the bridge is in there.
Note that that photo does not show the whole of the navigation.