Right Lads, next time the Gerry Marsden interview is on LFC watch it for full details.
However, we started singing it after it was in the Top 10 in 1963. The DJ at Anfield used to play the Top 10 before every home game & the Kop used to sing all the tunes, but when YNWA dropped out of the Top 10 the crowd carried on singing it & still do to this day. Celtic & I think Bruges from Belgium started singing it around the same time, probably for the same reason but couldn't give you a definitive answer.
I doubt the Catholic link is correct, cos I was brought up being told that Everton were the Catholic club!
The Glaswegians are very similar to the Scousers with their wit & you'll find they are visit each others city's a lot.