Donegal Castle
Today we went down the coast, following all around it's nooks and crannies for a while until we got to Dungloe and then we took the main road on to Donegal, with its castle (though still lots of coastline in that part too). We nearly missed the castle, in fact, because where Google Maps was telling me Donegal Castle was, was completely wrong - 20 minutes of driving wrong. Fortunately I spotted a sign before we'd gone far, and I submitted an edit to the place as well.
Donegal Castle is quite a nice balance of restoration and ruin, and though it is fairly small it's still well worth a visit. In fact it's so small I can fit the whole castle in with a single 24mm wide-angle lens!