The three Lakes of Killarney are separated by two narrow channels with bridges over them. When I visited the gap between the Upper and Middle Lakes under Old Weir Bridge was quite narrow, and the water was quite turbulent.
Back in 1998 the guidebooks and web sites all went on about "shooting the rapids under the bridge", and how it took nine men to haul the boats back up in the days before outboard motors. Looking online it looks like the channel has been widened.
Either that or if it doesn't seem like such an adventure now that we've all been white water rafting down the Nile. Either way, there doesn't seem to be any mention of it now.
The journey continues across the Middle Lake and into the wide expanse of Lower Lake, until eventually you round a corner and there is Ross Castle, and the coach waiting to take you back to your starting point. Or not in our case, as the chap was still digging up the road in Killarney, and everything was at a standstill.