There's few in Pasolini's version of The Canterbury Tales (1972). It's a rather odd viewing experience (even by Pasolini standards) as it features several well-known British actors who've been dubbed into Italian and then re-subtitled into English! I guess the obvious one for folksong in film would be The Wicker Man (1973) - some of them are original compositions by Paul Giovanni, but they all sound like the genuine article!