Rathfarnham Castle began as a late 16th-century fortified house, then grew layers of Georgian comfort and decorative ambition. The result is a useful Dublin counterpoint: less famous than the city-centre institutions, but good for seeing how defensive architecture became aristocratic domestic space.
The castle sits in south Dublin, so it is a deliberate detour rather than something you stumble across. It suits travellers who have time beyond the core sights and want a quieter historic-house visit with parkland nearby.