In Nestor Mr. Deasy tries to paper over the gap between his Protestant unionism and Stephen's Catholic nationalism: "We are all Irish, all kings' sons." The saying is proverbial, stemming from the medieval era of Irish history in which many separate regions were ruled by independent kings and everybody was a member of some ruling clan.
Deasy interprets the saying in an all-inclusive, self-congratulatory way consistent with his effort to convince Stephen that the present order is not only inevitable but ideal. In Proteus Stephen recalls the proverb sardonically, thinking about the many "pretenders" who aspired to control all of Ireland as High King, or to establish Ireland's independence from English overlords, or to establish a claim to the throne of England itself through some fabricated genealogy. In his view, the notion that every Irishman is a king's son is a black joke, born of the futility of powerlessness.