Hello
We are in Dublin for only 6 or 7 hours off a cruise ship,in April.
Are there any castles that are close that are open for tours close to the dock area and what's the easiest way to get there?
On The beach 51

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There are buses to get you into the city center. I would do a hop on hop off bus. There is Dublin Castle in the center of the city.
I have done this off a cruise ship, they should lay on shuttle buses into the city centre, usually Kildare Street.
We went first to Trinity College to see the Library (get there early to avoid queues), then got a hoho bus tour ticket, went rounfd the city and stopped off at Kilmainham Gaol, then back to city centre, even had time for a pint in a pub.
With such little time I would advise avoiding Dublin Castle, it's name is deceiving as it's not really a castle anymore. It is a centre of administration more than anything. You can do tours but they mainly consist of state rooms etc. Perhaps if you had more time you could add Dublin Castle to your list.
I like fushsiagirl's itinerary, you will get a good feel for Dublin and see what I consider to be one of Dublin's highlights, Kilmainham Gaol.
If you are dead set on seeing a castle you could visit Dalkey Castle, you will need to take the Dart from Connolly Stn, Tara Stn or Pearse Stn, it will take about 30-40 minutes to get there. This is really cutting it tight though and will leave very little time to see the rest/any of Dublin City. Purist will also tell you that Dalkey Castle is not a true castle, just a medieval fortified townhouse!
There is another contributor here who often recommends Drimnagh Castle, I have never been but his descriptions sound very good. You would need to take a bus there, and again, this would leave you with little time to explore Dublin City.
Best of luck.
Re. Drimnagh Castle, the Luas tram will take you from the docklands to the Naas Road and you can walk to Drimnagh Castle in about 15 minutes from there. There is however little else to do or see in Drimnagh
I would add that it would be very unusual for the ship not to lay on shuttle buses as you are not allowed just to wander round the port, they dont advertise this much in advance as they are trying to sell you their overpriced tours.
With that short of time, take a hop on hop off bus tour. There are 2 companies: 1 has red busses, the other green.
You will get a nice overview of the city & the history. There are no castles per se but there is plenty of amazing architecture including 2 cathedrals & you can see the old medevial wall.
The tour of the Dublin Castle is more like taking a tour of the WhiteHouse. It does not have a castle feel at all.