I think that they started singing that song (written by an Irishman, Paddy O'Reilly in the 80's, and not by Celtic fans) when Michael Owen got injured for a long time as it all about a character Michael being taken away.
"By a lonely prison wall, I heard a young girl cying, Michael they have taken you away etc etc"
It's probably the Fields of Athenray Dave, it's a Celtic song, a little bit like You'll never Walk alone, (which the Celts sang first). Scousers seem to like singing Glasgow Celtic songs for some reason.
The original words go something like
"Low, lie, the fields of Athenray,
where once we watched the small free birds fly,
our love was on the wing
we had dreams and songs to sing
it's so lonely round the fields of Athenray"
It's about an irishman, Michael, who gets sent to Australia for stealing food for his family. Nice tune, although I think the shell suited dole monkeys have changed the words.