Sitting in a great spot on a rocky headland where the Three Sisters rivers (that's the Barrow, Nore, and Suir) hit the sea at the mouth of Waterford Harbour, Duncannon Fort in County Wexford has amazing views for history and shipping. This isn't some pretty fairytale castle; nope, it's a tough, no-nonsense fort whose old stones seem to tell stories of takeovers and toughness. For hundreds of…