Here is the ESB’s Notifications page, with info on the rate of discharge from its hydroelectric dams and weirs. Today (14 December 2015) Parteen Villa Weir is discharging 440 cumecs (cubic metres per second or, roughly, ton[ne]s per second down the original course of the Shannon. That’s 44 times the 10 cumec usually discharged and more than replaces the 400 cumec diverted through the headrace to the Ardnacrusha power station. The Shannon is therefore running at its pre-Ardnacrusha levels and the Falls of Doonass have regained their power.
Of course if Ardnacrusha didn’t exist, its 400 cumec would be coming down the original course of the Shannon on top of the 440 cumec already there, which would make for interesting levels of flooding.
That ESB page has a link to this infographic, which shows the sort of information I was trying to get across here. I usually start from Leitrim [village]; the ESB starts slightly further upstream at Lough Allen. Note that the Shannon’s few locks are concentrated upstream of Lough Ree: between them and Killaloe are only two locks, at Athlone and Meelick, so the river’s fall is very slight.
Update 2018: the ESB has a new page with lots of interesting information here.
Irish waterways history 