Synge's In the Shadow of the Glen only appears to be simple or simplistic.  It is still timely today, but was extremely relevant at the time of its production.

According to the site I'll list below, here on enotes, several themes are present:

• The basic human need for security and the basic human need for freedom.  Nora marries for security, but her marriage brings her only loneliness and misery.  In the end, she--the only really likable and well-developed character in the play--chooses the life of the tramp over life with her husband.
• Feminism.  Nora is trapped, and miserable.  Again, she chooses freedom.  Neither of her choices is a good one, but the less lousy one is life with the tramp.
• Loneliness.  She learns from her husband that one can be with a person and still be lonely, while she learns from the tramp that one can be alone, but not be lonely.

The need for both security and freedom, and loneliness are always relevant.  And, unfortunately, though the world in some areas has made great improvements toward equality for women since Synge wrote, we still have a long way to go before it is achieved.