When this play was first written in 1907, it sparked riots in Ireland, as it was seen as blasphemous and insulting to Irish nationalists. The themes of the play include the value of rebellion. Christy Mahon commits the ultimate rebellion in his desire to kill his father, though he doesn't actually accomplish the deed. Peegan Mike, as the bartender's daughter is called, loves Christy for his rebellion and declares in grief at the end of the play, after choosing the more conventional Shawn Keough to wed, "I've lost the only Playboy of the Western World." The townspeople also celebrate Christy's supposed act of patricide, going against the dictates of religion and society.
The play also celebrates the value of freedom and unconventionality. As the play goes on and Christy develops as a character who is celebrated for his unconventionality, his speech develops a quality of poetry that he did not have earlier in the play. For example, when Peegan asks why Christy is lonely in Act II, he responds:
"It's well you know what call I have. It's well you know it's a lonesome thing to be passing small towns with the lights shining sideways when the night is down, or going in strange places with a dog nosing before you and a dog nosing behind, or drawn to the cities where you'd hear a voice kissing and talking deep love in every shadow of the ditch, and you passing on with an empty, hungry stomach failing from your heart."
Christy has transformed himself from a roughhewn country fellow into a romantic poet, and the freedom of being liberated from his father's control (or thinking he's free from his father) is the reason for his transformation. The play should be understood in the context of the times, when Ireland was considering freeing itself from English control, and the playwright celebrates the idea of shaking off one's family (or any source of authority) and living freely and unconventionally.
There are several different themes in The Playboy of the Western World by J. M. Synge. The first is gender relationships. The women in the play appear in some ways to be stronger than the men, and although admiring strength in men (e.g. Pegeen dumping the weaker Shawn for the stronger Christy) tend to usurp positions of domestic strength. The next theme is how the Roman Catholic Church seems to be complicit in building a passive character (e.g. Shawn). The next theme is how bullying gets passed down through the generation (Christy and his father). Finally, there is the sense that the "western world" is not the place of Celtic tradition and noble natives of popular imagination, but rather that oppression have built a character of cowardice and bullying.