I seem to recall an interview with PC on the radio. He shared a room with one of the Dubliners, if memory serves, and he penned the song in an afternoon. He played it for his friend and when he looked up he was crying... He said he knew then he had written a great song
Cannot find the reference though but I am sure I did hear this.
Learning it I could understand why. It's simplicity and devastating message make a powerful combination. I have sung it with Irish folks in the audience and was even thanked one evening for not forgetting the struggles.
Brilliant song and always touching, especially when played on piano in Coulter's version.