Big stillwaters like Lough Derg can be daunting but with the right weather conditions and approach catches can be huge. I have had many 100 to 200 pound plus catches in May, June and September on waters like this. Yes you need stable weather, ideally with a slight breeze and not too bright. But even in conditions like these I have enjoyed some great fishing for quality hybrids, roach and some bream (bream are naturally low light feeders).
Getting plenty of particle into the swim is critical (wheat is my bulk feed) with some added maggots, casters and chopped worm. You have to feed accurately, my killing zone is invariably 40 metres out into around 10 feet of depth, feeding and catching in around 5 square metres.!!! Clipping up on the feeder is essential. Derg usually requires boat access to the more remote deeper water swims. I never use a keep net if i want to fish the same swim on consecutive days.I only use one for my last day for the camera.!!!