What a great area for fishing you live in. I've had a few holidays in that region, mainly fishing the Erne but visiting several small tributaries. Some were of the deep canal type, others more stream-and-pool types. The fishing on the smaller waters always seemed to be either full on or full off. We'd often stop at bridges before deciding where to fish to check whether the fish were there or not. We tended to go in spring/early summer, so fish moving around in big shoals was typical. I'm not sure why fish would vanish in the way you describe in late summer, though.
As to the fish switching off in two distant swims at the same time, you're not the first to note strange coincidences of the sort. I was just reading a book by a well-known barbel angler, Trefor West. He recounts being amazed to find, one day in October, what looked like the entire barbel population - estimate 150 - of a stretch of the r|iver Teme congregated in one swim. It transpired that another angler discovered, on the same day, what looked like all the barbel from a stretch of a river 100 miles away, the Great Ouse, under a bridge there. More mundanely, I sat bream fishing without a bite for several hours on a lake. When I eventually caught and netted one, I looked over the far side, 80 yards away, and another angler who'd not moved all morning was netting one as well. Fish can do some stuff we can't explain.