I can't confirm the accuracy of my statement, Trudy. I found reference to the connection a couple of months ago on a Heritage Ireland site, but I have no way of knowing if the writer's research (if any) was accurate.

I just surfed for about 15 minutes and found references to four castles by this name. The one in the movie High Spirits, which is located in Limerick near Kildimo, is probably not the one in the song, as it is a relatively new castle dating from 1870.

I think the reference in the lyrics to the Blackwater (River?) might be a good clue. That would suggest the Kenmare location in Kerry, would it not?