How does The Thorn Birds end?

There are a series of events that mark the conclusion of the book.

First, Dane O'Neill, the son of Meggie O'Sullivan (née Cleary) and Ralph de Bricassart, dies in Greece. This event occurs shortly after he has been ordained as a priest in the Roman Catholic church. He drowns while attempting to rescue two women from drowning. His death has an important effect on the main protagonists. 

Cardinal Ralph de Bricassart learns that Dane was...

There are a series of events that mark the conclusion of the book.

First, Dane O'Neill, the son of Meggie O'Sullivan (née Cleary) and Ralph de Bricassart, dies in Greece. This event occurs shortly after he has been ordained as a priest in the Roman Catholic church. He drowns while attempting to rescue two women from drowning. His death has an important effect on the main protagonists. 

Cardinal Ralph de Bricassart learns that Dane was actually his son and he, too, dies (in Meggie's arms) after Dane's funeral.

The story continues with the fate of Meggie and Luke O'Neill's daughter, Justine. Justine initially ends her relationship with Rainer Hartheim because of her grief at the death of her brother, Dane. Eventually, however, they renew their relationship (after he visits her at the family estate, Drogheda) and they get married.