Green’s Dictionary of Slang

1. a bad, stale herring.

1910		P.W. Joyce Eng. As We Speak It In Ireland (1979) 307: Protestant herring; Originally applied to a bad or a stale herring.
1997		Share Slanguage.

2. any form of stale or unpleasant food.

1910		P.W. Joyce Eng. As We Speak It In Ireland (1979) 307: Oh that butter is a Protestant herring.
1997		Share Slanguage.

3. anything second-rate, inferior.

1910		P.W. Joyce Eng. As We Speak It In Ireland.
1997		Share Slanguage.