Some songs - like Wild Mountain Thyme - are not automaticlly traditional. It is the subject of much debate how much of Francis McPeak is in there. But he claimed it to be trad so it was put in the public domain by someone. BUT it is not always the case.. Lord of the Dance (lyrics) will still be in copyright in the UK because Sydney Carter, to my knowledge was still with us less than 5 years ago and hopefully still is.