Visits are free not sure about travelling though

It's a short walk.

There is no fee as the castle is only basement for coffee

To be honest I think staying in the city center is really the best thing to do as a visitor to Belfast. The Castle is very easy to reach with the local bus system or take a taxi.

There are no places set aside for sleeping on the passenger decks. May I suggest you contact the ferry company before bringing air beds etc and get written permission. If you don't you will likely be prevented from using them. One good thing is that footfall does decrease as the evening progresses so there should be plenty of areas where you can use your air beds etc. Mind you it's a long time since I slept on that particular ferry. so long ago it was the Ulster Prince.

The ground floor is open and there is a cafeteria in the cellars. There is also full access to the ornamental gardens. The upper floors weren't accessible whilst we were there but that may change as the rooms up there are normally available for hire so you should check on arrival. The staff are very friendly.

We went by private car so can't help with the public transport side of things. Barry 2137 has given you an excellent response. Alternatively, you can buy a pay as you go cellphone for around £10 ($13). You can use it for your time here and then dispose of it as you wish because there is no contract. Simply put on whatever funds you need while you are here. It makes pretty good sense because you just never know when you'll need to make an emergency call.

You can get a taxi anytime but they are not located near the grounds. You could book the taxi in advance, I am sure someone in the castle could arrange that for you. We are all friendly here and nobody will see you stuck!

Hi Castel is beutiful not sure how it looks at night timetime but summer time is getting dark late in Ireland between 8-9pm i think they should have some light on on the property :)