This site says the song was written by Charles O'Hegarty. [not Charles Hegarty] (and so does DT)
And This site gives some information about a folksinger named Charles O'Hegarty - don't know if it's the same guy - but it doesn't mention "The Body in the Bag."
By the way, something about this song annoys me. It doesn't explain why the cat looked dead. As far as I know, catatonia (sorry!) isn't a symptom of pregnancy.