As Ard Mhacha has pointed out, there are loads of operatic recordings of Marble Halls, either in full-length performances (or excerpts) of Bohemian Girl, or in albums of assorted arias.

But my whole point is, you don't need an operatic voice to sing this song - have a listen to Enya's recorded version of it! It's the sort of thing that suits informal treatment very nicely and is not hard to learn.