Why so many Siobhan McKennas?

I've read about this film for many years, and I've come to understand that it's much better than one might expect. I believe it was in BRITISH GOTHIC that I read the most interesting critique of it. Something tells me that Siobhan McKenna deserves a bit better than being almost last billed (is that due to her on screen credit being an introduction?) in the credits. And I wonder if all or some of the IMDB entries for Siobahn McKennas might refer to the same actress?