At time of writing, the depth of water at Athlone Weir is 1.7m. “Staff gauge zero is 35.360m above Poolbeg datum”, which means the water level is 37.06m OD (Poolbeg). According to Bob Cullen’s 2002 article for Inland Waterways News [PDF], “The minimum navigation level in Lough Ree is 36.88m OD”, so if the level drops another 0.18m, about 7 inches, there may be a WI Marine Notice.
It seems that the dropping of the level of Lough Ree is intended to provide a buffer against flooding in the area from Athlone downstream to Meelick. According to Brian Hayes “A meeting between the ESB, Waterways Ireland and the Office of Public Works to review the interim operating regime is due to take place shortly.” [I have asked Waterways Ireland for a report on the meeting.]
But, if we are to believe the Dublin Evening Mail, the Shannon Commissioners took a rather more robust attitude to flooding downstream of Athlone: steamers [and PR] came first.