O’Casey uses a variety of symbols in his play to highlight and reinforce some of his themes.
For example, in the opening stage directions we are told that "Between the window and the dresser is a picture of the Virgin; below the picture, on a bracket, is a crimson bowl in which a floating votive light is burning."
Although this seems like a typical symbol of Catholicism in a play in which religion is a theme, it is also used as a symbol to suggest important aspects of characterisation throughout.
Johnny’s anxiety, for example, that the votive light stays on seems to associate it with safety. The "crimson" colour perhaps links to his death later on.
At the opening of Act III we are told that the votive light "gleams more redly than ever" – the brighter red perhaps foreshadowing that Johnny’s death is getting closer.
Because of the link between the Virgin Mary and the mothers of the play, this brighter light could also foreshadow Mary’s news in this act that she is to become a mother.