The word you are looking for is 'Pishogue', which to quote Katherine Briggs' Dictionary of Fairies is 'an Irish fairy spell, by which a man's senses are bemused, so that he sees things entirely different from what they are in reality.' This does indeed make sense of the whole song,because as soon as the raven has been killed Michael sees thing as they really are, 'his true love with a wound so red'.

The song is nearly always sung incorrectly (after all how many people have actually heard of a pishogue?!!) and it was possibly my husband who originally spoke to you, Robbie. Thank you for listening, it's sad that such a lovely song is so often misunderstood.