Synge's The Playboy of the Western World reveals an Ireland in love with violence. This is the truth the play reveals. The characters in the play make a hero out of a man who kills his own dad. They put him on a figurative pedestal and celebrate his warped action. The people of Ireland have a warped sense of what to value.
And, of course, the hero doesn't even really possess courage or ability or anything of the sort. He doesn't even finish his dad off, and when the dad reappears, he cowers before him.
Ironically, the riots caused by the performance of the play weren't due primarily to the Irish being angry because they are portrayed as so violent and ignorant in the play. The riots were primarily the reaction to the swear words and rough language in the play. This, too, is revealing.
Synge reveals the truth about the Irish in Playboy just as Joyce does in Dubliners. Of course, the revelations apply to the rest of us, as well, not just the Irish.