Rangers signed a whopping 4 Catholics between 1917 and 1989. I could name them.
But, you have to be honest here, the Rangers board were very bigoted when it came to signing Catholics in this period and I guarantee that this played a role if a player got signed or not. They were not the only 'Company' during this time that wouldn't take guys on because of their bigoted mindset. My previous employer, Weir Pumps, who also had shares in Rangers, went through a long period of not hiring Catholics. It was well known. It might not have been written down for all to see but it definitely existed.