The Ancient Order of Hibernians (AOH) is an Irish-Catholic fraternal organization. Its largest membership is now in the United States, where it was founded in New York in 1836. Its original purpose in the United States was to assist Irish Catholic immigrants, especially those who faced discrimination or harsh coal mining working conditions. Many members had a Molly Maguire background. Its mixture of religion and politics (similar to that of the Orange Order) has been criticized by some as anti-Protestant. Members contend that this blend is necessary to defend against others considered to be anti-Catholic. In historical context, the Order may have emerged in America as a Catholic response to Freemasonry, which until recently prohibited Catholics from joining.

The use of the name in Ireland goes back as far as 1565^*, when it was founded by an Irish chieftain, Rory O'Moore, to protect Irish Catholics against persecution by English Protestants. It had an historical concept of itself as a continuation of the 1641 rebellion, a Catholic uprising which attempted to wipe out the Protestant Plantations of Ireland and to extirpate "heresy" (by which was meant Protestantism) in Ireland. These goals prevailed in its ranks into the 20th century, by which time it had developed into a militant lay-Catholic mass movement of Ribbon tradition.

At the end of the 19th century the AOH expanded hugely under its Grandmaster Joseph Devlin (later M.P.) of Belfast. The AOH was closely associated with the Irish Parliamentary Party, its members mainly members of the party. The AOH was against all secular idologies, such as those of the IRB, who in turn regarded the AOH as an old rival 'right-wing' nationalist society. As a vehicle for Irish nationalism, the AOH greatly influenced the ultra-sectarian course of Irish politics in the early twentieth century and by 1914 had saturated the entire island. It had been always vehemently opposed by William O'Brien M.P., as well as by his party, the All-for-Ireland League, due to their extreme opposition to any consessions being made to Protestant Ulster which could have enabled it accept an All-Ireland Home Rule settlement.