Looking specifically to stay in Ireland, then this is the directory you need to check out. Several locations listed and they are all gorgeous.
It shouldn’t be a surprise that some ancient castles would have a spirit or two. Castle Stuart in Scotland is particularly well-known for being haunted.
This site has a huge collection of potential castle hotels, as well as other mansions and palaces that can be rented. Not just the UK either, but she has locations all through Europe. Find castles in Spain, Germany, Italy or elsewhere!
Accommodations fit for a king, or at least a duke. These authentic castles in Scotland and Britain offer a unique hotel stay.
This index has a whole range of castle hotels through Britain, Scotland and Wales.
Look outside of the UK, and find some very unique castle accommodations in Spain instead.