Which Irish castles are the best visit? I want ones that are still standing with furnishings and deep history. Nothing built after 1850. Nothing that is only rocks. It doesn't matter if they are in Northern Ireland or the Republic of Ireland.
We enjoyed Cahir Castle.
Maybe Bunratty or Killarney? We prefer piles of rocks.
You could visit Belevedere House in Mullingar: https://belvedere-house.ie/ It was built in 1700's I believe. I am a bit biased as many women in my family were the servants for this estate so I have generations of history there. But it is a beautiful house, furnished with lovely grounds on a loch. Mullingar is about an hour west of Dublin.
We are heading to either Carrick Fergus or Dunluce (Causeway Coast drive, NI) later this year. The latter probably fits the pile of rocks category.
If we have time we will also visit Glenveagh in Donegal. Definitely not pile of rocks and furnished but its 1870.
I can't think of many places that fit your requirements.
Check out Bantry House in SW ireland.
Mount Stewart in Northern Ireland just slips in, being built in 1820.
Kilkenny Castle fits your description. The Butler House across the street was also part of the castle complex, and is now a hotel.
If it’s built after about 1500 it’s not a castle.
Ashford Castle is beautiful, you can see the entire castle if you are a guest. Or you could schedule an afternoon tea to see some of the main floor areas. History dates back to 1228, https://ashfordcastle.com/about It's about a 45 minute drive north of Galway city.
Check out Tales of Irish Castles on YouTube. It's a 6 part documentary castles throughout Ireland. Great historical background. Make sure that you watch them in order. Host is Simon Delaney and he is very entertaining!