Mo Johnston is the only one I can think of, as he was the first catholic to play for Rangers. I also think he went to someone else in between.
Kenny Miller more recently, but he was at Cardiff as well.
Just looked Mo up, he went to Nantes in between. He also played for the other big Glasgow team, the Jags (Partick Thistle).
Geez just noticed Partick Thistle.
Few of my Ancestors played for them.
Just for interest .........
John Proudfoot was a centre-forward who played for several seasons, making his debut against Dundee Wanderers at Inchview on 11/08/1894, when he scored two goals. Brother of David, George, Lawrence and William who all played for Thistle. On leaving Thistle he joined Blackburn Rovers, scoring on his debut and playing 37 times, scoring 14 goals (top scorer in 1897-98). He left Ewood Park for Everton where he scored 30 goals in 84 games (top scorer in 1898-99) before he joined Watford. After being suspended for "insubordination" in December 1902 he returned to Thistle for a season before ending his career with Hamilton. Worked in the shipyards of the Clyde.Career: Partick Thistle 1894; Blackburn Rovers 1897; Everton 1898; Watford 1902; Partick Thistle 1903; Hamilton Academical 1904.Representative honours: Glasgow v East of Scotland 1894 (cap below); v Sheffield 1895John was also capped three times when he represented Glasgow in city versus city games, two of which were against Edinburgh and Sheffied.