From 26 June through 1 September 2017, ESB will be providing guided tours of Ardnacrusha hydroelectric power station. It’s not usually open to the public and, if you haven’t seen it, it’s well worth a visit. Go here to book.
I have some information about the site here, here and here, but you should try to see it for yourself.
Update July 2018: tours available again this year.
Irish waterways history 