The cache is not at the posted coordinates, which are in the middlle of the River Foyle - you will get wet!!
To find the final container you must have completed the A Peaceful Walk Adventure Lab Cache, which involves a pleasant walk across the Peace Bridge.
You will need to download an Adventure Lab app to complete the adventure. These are available for IOS and Android through the links.
To access the adventure, click on the link A Peaceful Walk or open directly in the Adventure Lab app
At each of the five locations you will have collected a code. Enter the 5 numbers from A=E in the format 12345 into Certitude