Arthur O'Sullivan (also credited) seems to have been a well known Irish actor. However his "official" film resumé leaves this film out. Maybe the actors who worked on the film collected their paychecks and thought the film had never been released. A lot of it does seem to have been shot in Ireland, the interiors as well.
And yes, it does seem likely that quite some of the supporting cast is Irish rather than Italian.
Dagmar Lassander told me that the film was shot in Ireland, she was there when they did, and also it was shot by Riccardo Freda, as some places have claimed that he didn't direct the film (and thats why he used the name Willy Pareto for the film). Also there is a rumour that Freda said he didn't direct one of his films in the 70s - but which one? Not this one? And not Tragic Ceremony - because Camille Keaton remembers him.