Green’s Dictionary of Slang

a Roman Catholic.

1871		R. Rowe Episodes in an Obscure Life 16: She was very much disappointed when she was told that the Great Fire after all had not been caused by Roman Catholics. [...] I can’t abide them wild Hirish [...] there’s Blue Anchor Court close by the Rents as is full a’ Romans.
1876		Besant & Rice Son of a Vulcan II 179: She was a Roman, poor Biddy. I’m a Prodesdan.
1919		E. Dyson ‘The Single-Handed Team’ in ‘Hello, Soldier!’ 102: He’s Roman, I’m a sort of Proddy.
1956		‘Ed Lacy’ Men from the Boys (1967) 22: I’ve known some pretty good nigger cops and Jew-boys. And Bill Ash is a Roman.