prayers cannot defeat the power of the seemingly supernatural sea.

At the end of the play, Maurya says, "They're all gone now, and there isn't anything more the sea can do to me. . . . I'll have no call now to be up crying and praying when the wind breaks from the south." In other words, by the end of the play, it is clear that the power of the supernatural that is invested in nature is more powerful than religion. Maurya has no more reason to pray, as it's obvious that the supernatural can defeat her prayers.

In large part, the whole play is about reconciling the relationship of the natural to the supernatural.  In the dangerous natural environment of the Aran Islands, where the sea is a constant threat to the fishermen’s lives, the inhabitants must reconcile their real-world lives with their beliefs in the “supernatural.”  In the real, natural world, a drowned sailor’s body washes up on shore and is identified by his distinct knitted sweater.  In the supernatural world, he is delivered to his family by riding a pale horse, a universal symbol for death.  The family, which has already lost members to the sea, sees as inevitable this tragedy, because life itself on the Aran Islands is a battle with natural elements.  The term “reconcile” indicates this acknowledgment, and gives the play its poignant tone.

As the previous poster noted, one of the main themes in the story is that of the sea.  Because her sons are always leaving her to be killed by the sea along with her husband, Maurya has become terrified of its power and wants desperately to find a way to save her two remaining sons from its power.  In the end, however, she cannot do so and the final two end up drowning and the incredible power of the sea both as an attraction and a destroyer is made evident.

Another theme is that of loss and the inevitability of it.  Of course her sons and her father did not have to go to sea and not everyone who goes to sea drowns, but Maurya finds a sort of fate in the power of the sea over her family and her dreams and her waking life are haunted with the power of the sea to reach out and take away her most precious possessions.

One of the major themes of Riders to the Sea is the inexorability of fate. That is, fate is inevitable, and there is nothing humans can do to stop it. At the beginning of the play, the matriarch of the family, Maurya, hopes she can save her last remaining son from drowning in the sea as all of her other sons and her husband have done. Despite her efforts to stop him, her last son goes to the sea and drowns. Maurya, though bereft, now knows that there is nothing left for the sea to take from her. A related theme is the powerlessness of humans, including religious figures, to combat the power of nature and of fate. Even the priest in the village cannot stop the sea from claiming Maurya's sons and husband. Religion is powerless in the face of nature and fate.

"Riders to the Sea" by John Synge is a play about peasant life in the Aran islands of the western coast of Ireland. The play is not overtly ideological, but does give a heart-wrenching vision of the lives of poor, Roman Catholic Irish fisherfolk, who are dependent on the sea for their livelihood. 

The first major theme we encounter is the implacability of nature. Eight men in the family have died from drowning. Synge shows how this experience, typical of the area, shapes the religious and philosophical beliefs of the characters in the play, leading them to a sort of stoical fatalism. Although they are portrayed as turning to religion for solace, religion is not shown as actually offering consolation, but rather encouraging resignation to their fate and a sort of passivity in face of their struggles. 

Another major theme we find is an association of women with endurance in the face of suffering. 

A related theme to the one just mentioned is that of the tension between paganism and Catholicism in this remote Irish fishing community. Like everyone else in the village, Maurya is a Roman Catholic. Yet despite this, she places more faith in the movement of the stars and other natural phenomena than she does in the glib platitudes of the village priest.

Maurya has never lived anywhere else than on the island. She understands how the place she has always called home is indissolubly shaped by the elements. In turn, she and the other inhabitants are shaped by their environment to the extent that their whole mindset is largely determined by the features of the surrounding landscape: the rocks, the beach, and the clashing waves. These elemental forces of nature are far more real to the villagers than the formal teachings of the Catholic Church, to which they pay little more than lip-service.

The dominant theme of Synge's, Riders to the Sea, is fate.  The tragedy plays off of ancient Greek tragedies which also deal with fate, such as the tragic drama of Oedipus.  (The name of the character, Maurya, for instance, is a variant of the Greek word for tragedy.  The play opens with Cathleen spinning at the spinning wheel just as the Fates do in Greek myth.)  Fate in the play is determined by the sea.  The Irish peasants are helpless against the natural force that every male must contend with in order to provide for himself and his family:  the sea.  The sea is more powerful than God and the priest, and it alone determines the peasants' fates.  There are only two riders to the sea in the play, one already doomed and the other soon-to-be doomed. 

One might be tempted to elaborate and speak of Irish poverty, Irish rural life, the love between a mother and her men, etc., but essentially Synge mixes Irish folklore with Greek myth to create a tragedy that reveals natural forces dominating the fates of helpless humans.

We are presented with a world in which both natural and supernatural elements seem to coexist. The isolated island on which Maurya and her family live presents itself as a perfect place where superstition and religion have a massive impact on its inhabitants, and this can be seen principally through the character of Maurya and the way in which she sees a vision that predicts her last son's death. Note what Maurya says after going to give her blessing to Bartley:

I've seen the fearfulest thing any person has seen since the day Bride Dara seen the dead man with the child in his arms.

We are plunged into a world where blessings are important to avert evil, and where visions are common and talked about as foreshadowing important events. Maurya's vision of her dead son Michael with Bartley effectively shows how Bartley as well will join his brothers in being lost to the sea that will leave Maurya destitute.

In writing your essay for this prompt, I would also suggest that you consider the sea as an example of uncertainty, especially the uncertainty of rural life in Ireland at the beginning of the 20th century. As the other answer to this question points out, the sea is both a giver of life (in that it supports fishing, trade, and general commerce), but it is also a deadly force and often drowns those who travel on it. Thus, in addition to exemplifying a cycle between life and death, J.M. Synge's representation of the sea also exemplifies the uncertainty of rural Irish life. The elements and forces that sustain a community occasionally also destroy said community, and so the rural life Synge is depicting is a constant balancing act and never wholly secure. By extension, Synge's exploration of the tumultuous sea reminds us that the security and safety of civilization is often an illusion, and that we are all actually at the mercy of the uncertain forces of nature, just like the main characters in Riders to the Sea.

If you're looking for books on the play, I might suggest looking at Nicholas Grene's Synge: A Critical Study of the Plays. As a former student of Dr. Grene's, I can attest to his groundbreaking work in the field of Irish drama and Irish literature in general. Additionally, it's worth checking out eNotes' helpful list of books and sources on the play, and I've provided a link to these resources below. 

The sea is one of those universal symbols which is not decoded by one culture much differently from another as it is a common reference to all. First, it represents the source of life, the primeval origin of all creatures; it is the "womb of the world." Antithetically, by its sheer strength and power of destruction it represents death as well. Thus an everlasting "womb-tomb" cycle takes form, with both life and death forces taking equal part.

This idea comes over very strongly throughout this play. The sea is both the source of substenance for the family and the reason for its destruction, as one by one Maurya's sons give up their lives to the sea. There is first pathos created by the senseless fatality of their deaths, then resignation to the inevitable.

Check out the following references to glean useful information for your research. (Be sure to navigate through the subtopics as well.) You might also want to check out naturalism as a literary tendency, in which the forces of nature are considered destructive and wilfully hostile against man.

Synge's Riders to the Sea is a Greek-like tragedy set among Irish peasants on the Aran Islands.  The fate of the peasants rests with the sea and is out of their control.  The men on the island have no meansof providing for themselves and their families except the sea.  And the sea is unmanageable and destructive.  The men will try to earn a living on the sea, and they will be killed.  That is their fate.  The women will be left alone with no way to provide for themselves.  The peasants are trapped.  That is their fate.  The play closes with the mother, Maurya, giving in to fate and stating that she now has nothing left to lose, since everything has already been taken from her.