The Informer (1935)

Dublin, 1920. Gypo Nolan, strong but none too bright, has been ousted from the rebel organization and is starving. When he finds that his equally destitute sweetheart Katie has been reduced to prostitution, he succumbs to temptation and betrays his former comrade Frankie to the British authorities for a 20 pound reward.

In the course of one gloomy, foggy night, guilt and retribution inexorably close in.