I’m currently investigating the Rockville Navigations. Hugh Malet, in his book In the Wake of the Gods (Chatto & Windus 1970), describes a trip in a small boat on these navigations. He and Kay, his wife, started from Grange in the Carnadoe Waters (off the Shannon above Roosky) and had to work the boat up the shallows in the river. They got into deeper water and travelled through a series of small lakes, linked by some natural and some artificial channels.
Harry Rice shows the navigations on a chart he drew in 1960; there is a copy on the Heritage Boat Association website here. And there is a brief mention in one of the early Shannon guides. But apart from that, the navigations seem to have been forgotten, although some people have boated on parts of the system.
The navigations are shown on the early Ordnance Survey maps from around 1830, and the centre seems to be at Rockville House, which was owned by a branch of the Lloyd family, owners of thousands of acres in County Roscommon. If Hugh Malet was right, the main cargo was turf.