Seven hundred years of English rule administered from one courtyard, and almost none of it looks like a castle. The Viking foundations underneath are the best part.
Not the castle you are picturing
Almost nothing of the medieval fortress survives above ground. A fire in 1684 destroyed most of it, and what stands now is largely eighteenth-century Georgian administration building arranged around two courtyards. The Record Tower on the corner is the one substantial medieval survival.
That mismatch between name and appearance disappoints people who arrive expecting battlements. It is also the point: this was not a defensive castle for most of its life, it was the seat of British administration in Ireland, and it looks like a government office because that is what it was.
The Viceroy lived here. The Irish executive worked here. The 1916 rebels considered taking it and decided against. It was handed over to Michael Collins in January 1922, and the anecdote that a British official complained he was seven minutes late — to which Collins is supposed to have replied that they had been waiting seven hundred years — is almost certainly invented and gets told anyway.
What to see
The State Apartments — St Patrick’s Hall with its painted ceiling, the Throne Room, the Portrait Gallery. These are still used for state functions, including the inauguration of presidents, so rooms close at short notice.
The Chapel Royal, neo-Gothic, 1814, with plaster fan vaulting and over ninety carved heads on the exterior.
The Undercroft is the reason to take the guided tour. Below the courtyard are the foundations of the Norman tower, a section of the Viking-era city wall, and the point where the River Poddle — now culverted — filled the moat. The dark pool the Poddle formed here, dubh linn, is where the city gets its name.
Around it
The Chester Beatty Library is in the castle grounds, free, and is one of the best small museums in Europe — see its own entry.
The Garda Museum in the Record Tower and the Revenue Museum are both free and both small.
Dubh Linn Garden behind the castle is a quiet circular lawn with a helicopter landing pattern cut into it, on the site of the pool itself.
Practical
Guided tours run through the day and are the version to take. Buy on the door or online; it is rarely sold out.
The courtyards are free to walk through, and they are a useful shortcut between Dame Street and the cathedrals.
More Landmarks
Trinity College and the Book of Kells
A walled university in the middle of the city, founded in 1592, with a ninth-century illuminated gospel book and a library that looks the way libraries are supposed to look.
Ha'penny Bridge
A cast-iron footbridge from 1816 that charged a halfpenny to cross for a century. Free now, and the most photographed object in the city.