When the archaeology group is singing around the campfire, it is clear that the original song that the actors were singing before the dub is not "Molly Malone." The tune being played on the guitar has nothing to do with the song being sung, and the timing of the singing to the guitar is wildly off.
Opening Credits: a [sic] ETTORE SPANGUOLO film.
When the butcher is entering the basement of the monastery, it looks like he is wearing a backpack or something underneath his jacket. Look at his back as he turns around a few times when he has walked down the stairs. Suddenly it's gone and not seen any more.
In the opening scene, as the nuns are being taken down the stairs, it's obviously a time prior to the modern era, as the men are all holding lit torches. There is, however, a large suspended electric light visible above their heads (right above the crosses).
When Porter arrives at his boat at night he sees that the lights are on but swears that he didn't leave them on. He walks below deck on his boat and when he walks down the stairs the shadow of a crew member or a camera is clearly visible above the stairs.