Thank you Declan. You've nearly convinced me that I'm not entirely mad (another source of debate). I too am not a gaelic speaker, and most certainly couldn't claim any expertise on the subject, but have heard pishogue/pishog/piseog pronounced with an 's' or 'sh' sound.
Perhaps a gaelic speaking mudcatter could give some info on the correct pronunciation or possible regional variations.
Having sung the song myself and had this pointed out to me, I found that both re-listening to the record and looking up the meaning convinced me that this was the correct word.