Hello.
I posted the original picture, and today was the first alert I'd received about request for info. so apologies for any delay.
The mine entrances are on the 'left bank' of the river. There is the one pictured plus another lower down, almost at the river level, which I didn't explore. If you know the area, take the footpath that follows the river through the grounds of Culloden Tower (from next to Yorke Square car park), heading upstream. You can't miss the entrances.
I have been in the adit on the opposite bank with a steel gate; it was on a geology course, and the key must be available if the right person is asked!