Who are the antagonist and protagonist in The Thorn Birds by Colleen McCullough?

The protagonist is Meggie Cleary. Even though there are a lot of characters and the story spans several generations, Meggie is the most central character and her plot (her impossible love for Father Ralph de Bricassart) is the most heavily weighted.

There are different kinds of plots in story-telling, and all revolve around a conflict. Sometimes the plot conflict is between one character and another, the protagonist and the antagonist, but quite often, the protagonist...

The protagonist is Meggie Cleary. Even though there are a lot of characters and the story spans several generations, Meggie is the most central character and her plot (her impossible love for Father Ralph de Bricassart) is the most heavily weighted.

There are different kinds of plots in story-telling, and all revolve around a conflict. Sometimes the plot conflict is between one character and another, the protagonist and the antagonist, but quite often, the protagonist is in conflict against something other than another character. The conflict could be against nature (for example, lost in the woods or fighting a bear), society (for example, on the run from the law or fighting for civil rights), a machine (such as a giant robot or trucks that come to life), or even him/herself.

In the case of The Thorn Birds, Meggie's greatest antagonist is herself. She makes decisions that lead to problems, and then she has to search for solutions. For example, when Ralph chooses not to destroy the secret will, inheriting Mary's 13 million pounds for the church, Meggie could say, "Oh, Ralph chose money and power over me. I guess he does not love me strongly enough. I should forget him and move on." Then Meggie would probably meet a good man and have a pretty decent life. But there would be no story. Instead, Meggie pines for Ralph and marries Luke, who treats her like property and isn't a good companion or husband to her. So Meggie's main antagonist is her own thoughts and feelings, and her conflict is mainly internal.

Meggie and Ralph also have some conflict with society; specifically, the rules that the Catholic religion has in place to govern morality. According to the church, priests are not allowed to marry. They should be fully devoted to God and married to the church. Therefore the morals of the church can be considered a secondary antagonist in Meggie's story as she struggles through life in love with a man she can never have.