Conor Larkin, the protagonist of Leon Uris' Trinity, is an Irish Catholic. The novel itself is set in the early twentieth century, a period in which Ireland was fighting for independence from England, with exposition tracing the earlier history of the English conquest of Ireland and its consequences.
The vision of Roman Catholicism we have in the novel is one which links Catholicism to Irish patriotism, mainly because the English under Henry VIII, who fully subdued Ireland, were Protestant and Anglo-Norman in heritage, while the Irish Celts were Roman Catholic. Even though historically, Roman Catholicism was imposed on the Celts (who were first pagan and then adopted Celtic Christianity, which differed from the Roman model in several details), especially after the English conquest, Catholicism became a form of nationalism and indigenous opposition to England. The importance of Catholicism in every aspect of Irish life is seen in the following quotation from Trinity, which expresses the degree to which even the Republicans could be frustrated by the degree to which religion had become a focus of Irish sentiment:
The only time we can attract a crowd is for some pilgrimage up some god-damned holy mountain to chase the snakes and banshees out of the country.
The novel emphasizes how the Protestant Ascendancy monopolized power and wealth in Ireland and the degree to which the Irish Catholics were oppressed.