What is your typical travel style? And what is the travel style of the people that you are going with?
I have only travelled with a max of three other people before, and I'm usually solo - so take this advice as you will. I am someone who likes to do things spur of the moment with little to no planning whatsoever, I'm also quite frugal.
Where in Ireland are you going? When I went, my destination was Cork, so I flew into Shannon (which was much cheaper than other airports) and took a shuttle.
One thing that I wished I had researched before I went was the proximity of the cities, because I'm used to being able to hop on a train and go visit a random town for a day and then return to wherever I'm staying. You can't really do that in Ireland, because the cities are more spread out. There are day trips that you can take, however. I used
Bus Eireann and did their Ring of Kerry trip, and also took the bus to Blarney and hung out there for a day.
I stay in hostels or apartments when I travel because, as I said, frugal. I'm not sure about how much a castle would cost or what the wait list for something like that would be. But there's the site
Celtic Castles which may be useful for that.
You may also want to look into renting a car and driving while you're there. With a group that size, it might be your best bet.
Hope this helps!