Some preparation and Web research will be essential to begin with, as the coordinates are just for a convenient parking area. The second stage is a very tough test: only the determined will succeed.
The first part is an easy and interesting little stroll (less than a mile on good surfaces), which should take an hour or so.
From there, go to N 54° 04.ABC, W 4° 45.DEF. Use a map to find the location.
If you aren't confident about the coordinates you've gathered, check using this link: Geochecker.com before continuing.
Approach this location from the NE (along the coast: in winter and spring you may find a shorter and slightly easier approach from the west, but the bracken is head-high and impassable by late June). This is a very difficult scramble on slippery rocks and wrack: although low lying, there is no path - don't attempt the approach when the weather is wet or very windy. Don't go alone (at least take a companion part of the way). Ensure that the tide is well out - it should only take half an hour, but allow an hour for the return trip to the road. Take a walking stick, notebook and pencil, and a torch. Wear sturdy clothing in summer as there will be bracken, nettles and brambles around the pit. The pit entrance is small and very well hidden: so even if you think you've guessed roughly where to look, you'll need to use the GPS or you'll be scrambling about all day. Shine the torch into the darkness to see the pool. Don't climb down into the pit (and be careful not to slide into it): there's a blue tape sling attached to a gorse stem near the entrance, and you just need to pull on this. Be prepared for what you haul up...then lower it back down carefully out of the way of muggles.
Return to the road, and on to N 54° 03.GHI, W 4° 45.JKL. This final stage is on easy ground, and you can get to the cache without having to climb over any fences or walls. Take care, and good luck!
The box is a smallish plastic one, so don't bring large swaps. Original contents;
North Carolina Geocoin
TT 2001 badge
Peel Castle Puzzle cards
...and for the FTF...
Darrach's Antique Gold Cufflink and Tie Pin Set