The batteries were built and staffed by 1814, the year before Napoleon was defeated at Waterloo, which effectively made them obsolete, and thus they never served their intended function. This particular one in recent times was repurposed as a barn.
This area is a favourite of fishermen, and locals often go swimming in the little inlet directly in front of the battery. However as there is no ring buoy on hand, and as the currents can be quite strong at certain tides it is probably a better idea to swim at "The Long Wall", another popular swimming area that you will have passed on the way (easily identifiable by the long, low wall by the road).
This is one of my favourite places, with views of the Shannon Estuary and Scattery Island. If you have good eyes, or perhaps a pair of binoculars, and the day is clear you can see the Scattery Island Battery from here.
It is also possible to see seals at this point, although I wouldn't get your hopes up too much, as it's rare enough to spot them. But it does happen.
The cache container is a plastic jar, containing a logbook, a whistle, and a drop of Guinness (they used to give these out when you took the tour of the Guinness Brewery, but those days are gone) and has room for travel bugs and geocoins. In my eagerness I forgot to put a pencil in, so best bring your own until I rectify the situation.
The cache can be approached over the rocks, but I would advise going through the field as there is a trail through it that is popular with walkers. Please stick to trail, respect the land, and keep dogs on a lead at all times, as there can be cows in neighbouring fields. CITO-ing while you're here would be great as well!
Be careful with junior cachers along the trail as there are some steep drops on the seaward side. Of course care should always be taken by the water, and if you do decide to go down on the rocks, be extremely careful as the rocks can be very slippy, and the water is deep.