I came across this article, which is about the origin of Ireland's flag. As you may know, they had a lot of Catholic vs Protestant problems in the past, and certainly concurrent with when FH was writing Dune.
I have always wondered at the exact etymology for "Orange Catholic." For a while I wondered whether Burgess' book A Clockwork Orange might have had some relevance, but it turns out it doesn't. The explanation behind the Irish flag makes much more sense: that orange was the color of Irish Protestantism (and of a minority cause at the same time), and in naming it the Orange Catholic Bible I think FH is suggesting that at some point the schism between Catholicism and Protestantism was resolved. "Orange Catholic" would be like saying "Protestant-Catholic". Maybe there's even some connection here in that the Atreides banner color is green. Or maybe a coincidence?Meagher and his fellow Young Irelanders had been inspired by the 1848 revolutions across Europe. In April of 1848, a contingent of them traveled to France to congratulate the rebels there on overthrowing King Louis Philippe I. There, Meagher was presented with an Irish tricolor woven out of French silk.
Upon returning to Ireland, he, in turn, presented it to the Irish people, explaining the symbolism of the flag’s three colors: “The white in the center signifies a lasting truce between the orange and the green,” he said, “and I trust that beneath its folds the hands of the Irish Protestant and the Irish Catholic may be clasped, in generous and heroic brotherhood.”
The green represents Irish nationalism; the orange, Ireland’s Protestant minority, and the Orange Order; the white, lasting peace between the two.