Green’s Dictionary of Slang

(Irish) a Catholic.

1963		L. Daiken Out Goes She 16: Catty, Catty go to Mass / Ridin’ on the Divil’s ass / When the Divil rings the bell / All good Catties go to Hell.
1975		W. Trevor Angels at the Ritz 171: ‘Proddy-woddy green-guts,’ the Catholic children cried at us in Curransbridge. ‘Catty, Catty, going to mass,’ we whispered back, ‘riding on the devil’s ass.’.