What makes Dublin Castle historically important?
Dublin Castle occupies a shallow rise in the medieval core of Dublin — the same strategic promontory the Vikings chose for their fortification and the Normans subsequently reinforced with stone — and served as the administrative headquarters of British rule in Ireland from 1204 to 1922. What visitors see today is almost entirely Georgian and Victorian, the original medieval fortress having been demolished and rebuilt as a palace-cum-government complex during the 18th century, but the Record Tower (ca. 1258) and the Powder Tower Undercroft preserve the medieval skeleton beneath.
The castle's Georgian State Apartments — the Throne Room, the St Patrick's Hall (Ireland's largest and most ceremonial room, with banners of the Knights of St Patrick hanging from its ceiling), and the Wedgwood Room — were used by British Viceroys for entertaining, ceremonial investitures, and state occasions. They are now used by the Irish President for similar purposes: EU summit meetings, presidential inaugurations, and state receptions all take place in the rooms that once hosted the machinery of colonial rule. The Chapel Royal (1814), a neo-Gothic confection by Francis Johnston with extraordinary plasterwork vaulting and over 100 carved heads of English monarchs and Irish saints on its exterior, stands in the castle's Lower Yard.
The Chester Beatty Library in the Clock Tower Building deserves as much time as the State Apartments — arguably more. Its collection of world manuscripts and decorative arts, gifted to the Irish state by Sir Alfred Chester Beatty in 1968, is displayed with exemplary scholarship and imagination, and the free admission makes it one of the most generous cultural gifts in Ireland's heritage landscape.
In the castle's Lower Yard, a plaque marks the approximate location of Dubh Linn (the Black Pool) — the tidal inlet from which Dublin took its name, now drained and built over but still the etymological root of the city's identity.