I've no idea when I first heard "Johnny Todd" but it was a favourite of Brian Kelly's and the version I now sing is a transcription of his rendering. It is also a penny-whistle favourite of mine.
It is a typical sort of sea song on the unfaithfulness of the ladies when their husbands and sweethearts are away at sea. The last verse gives practical advice in its proposal that a sailor should marry his intended before sailing away. What stuff and nonsense!