THE CACHE IS NOT AT THE POSTED COORDINATES - INDEED YOU MIGHT GET VERY WET IF YOU GO THERE
To locate the bonus cache you will have to complete the Iconic Art - Belfast Adventure Lab Cache.
You need to have or download an Adventure Lab app to explore and complete the adventure. The app is available for IOS and Android here.
To access the adventure click on the link or scan the QR code below:
Iconic Art - Belfast - Bonus
At each stage an image will be revealed in your journal including information to find the cache location.
The cache is located at: N54 3(A-E). (C+1)(A+2)(C-D) W005 5(F-1). (A-C-1) (B-C-3)(B-4)