Several organisations maintain sensors that detect water levels; some of them publish their readings on tinterweb.
The Office of Public Works, for example, shows real-time water levels here; this page shows the changes at Banagher over the past thirty-five days. It may be that the ESB’s run-off of water at Parteen Villa Weir has been having some effect.
The Environmental Protection Agency has a list of monitoring stations but you have to swear not to burn down the queen’s dockyards, insult the president or sell your granny before you’re allowed to look at it. When you’ve done that, you can view Hydronet data, but you have to choose your River Basin District first. If you choose the Shannon RBD, you get a list of stations — none of them on the Shannon itself — arranged in no comprehensible order, so you’re on your own after that. Here’s the one at Tyone on the Nenagh River by way of example.
Finally, Waterways Ireland has information here. The page takes quite a while to load. It covers only the waterways for which the body is responsible and information about the current water level (as compared with MSL Malin) is of limited use unless you’ve been monitoring it for some time. And, alas, there is no information on anywhere on the Shannon south of Meelick (Victoria) Weir, presumably because Waterways Ireland can’t control anything south of that.
Who can? The ESB, and I suspect they must have gauges at, say, Killaloe, but if they do I can’t find the readings published anywhere. It would be a boon and a blessing to men if ESB were to publish its information.
There is one other source of information that might help: there are some webcams on Irish rivers. Farsons have several, though none on the Shannon, and at the moment I can’t get any of the Irish ones to work for me.