According to the Met Éireann inland lakes forecast [are there outland lakes?], Lough Derg is currently experiencing southerly winds of Force 3 or 4.
According to Windguru, it’s F4 but with gusts to F6 or F7. XCWeather agreed on F4 gusting F6; Windy says F4 gusting F7.
Messrs Windguru, XCWeather and Windy seem to be closer to reality than Met Éireann. It may be forecasting the base or average wind speed, but anyone going out there now, based on a forecast of F4, is going to be seriously discommoded.
Irish waterways history 