Grannagh Castle stands on the Kilkenny side of the River Suir, just outside Waterford, where its ruined walls still command the river crossing. The remains are atmospheric rather than polished, with enough surviving masonry to make the defensive position and medieval river traffic easy to imagine.
It is a short add-on for travellers with a car or bicycle rather than a full destination by itself. Pair it with Waterford's Viking Triangle for a broader sense of how the city and its river approaches were guarded, traded through, and contested over centuries.