The Defenders (Irish: Na Cosantóirí) were a popular, oath-bound, society that reflected the range of Catholic grievance in late 18th-century Ireland. First formed as a vigilante response to the violence with which Protestants resisted competition for tenancies and employment in County Armagh, the Defenders spread rapidly as a lodge-based based fraternity across Ulster and into Leinster and Dublin City. A campaign for Catholic emancipation that excited hopes of tax and rent relief, and resistance to militia conscription, broadened and politicised their membership. By 1796, the Defenders had allied with the United Irishmen, republicans who, making common cause against the landed Anglican ascendancy, drew them into the rebellion of 1798.

In 18th-century Ireland, there was a "varied, energetic and complex structure of agrarian 'secret societies'", commonly referred to as Whiteboyism, after groups that had emerged mid-century in the Munster. Employing elaborate oaths and ritual, they bound tenant farmers and cottiers in covert acts of resistance to the rack-rents, tithes, and taxes levied by landlords, the "landlords' church" and county Grand Juries.

In the north, in Ulster, the phenomenon could be found (in the Oakboys and Hearts of Steel) among Protestants, particularly among the Ulster-Scot church "Dissenters", the Presbyterians. Whiteboyism was otherwise the protest of Ireland's dispossessed and penalised Catholic majority. While this was case for the Defenders, from the outset they represented a "sectarian alliance" broader than a conspiracy against process servers, tithe proctors, and tax assessors.

The Defenders emerged in 1780s from a contest in mid-Ulster not, in the first instance, with landlords, their allies and agents, but with Protestants competing for the same tenancies and cottage employment. As a result of a relaxation of Penal Law restrictions on Catholic lease-holding, and of an influx of refugees from the intense poverty of Ireland's western counties, Protestants in County Armagh found themselves increasingly outbid on plots of land when their leases expired, and undercut in the already-depressed market for home-spun linen. On the pretext of searching for arms, prohibited to Catholics under the Penal Laws, some would respond by raiding Catholic homes, smashing spinning wheels and turning families out upon the roads. It was in the wake of one such night-time foray by the "Peep o'Day Boys" that, in 1784, the first posse of Defenders was formed near Ballymacnab. Supplied with arms illicitly purchased from a Protestant shopkeeper, local men embarked on night-watches and patrols.

Confrontations with the Peep o'Day Boys extended to fights at fairs, markets, and races, with Protestant magistrates and juries typically finding against the Defenders alone. In 1788, Lord Charlemont re-organised Armagh's existing Volunteer militia into new companies expressly committed to Catholic exclusion and to pan-Protestant unity. Volunteers from these companies moonlighted as Peep o'Day Boys, escalating the violence, and contributing to its increasingly indiscriminate sectarian character. To quell the "Armagh disturbances", in 1789 the government sent in the military, but the under cover of darkness the violence continued and spread across county lines.

By 1790, Defenders were active across Ulster and the bordering counties in Leinster. Borrowing from the ritual and practice of Freemasonry, they organised in lodges, federated under county Grand Masters and committees. They had also taken a first step on a political path: their varied oaths, committing members to mutual assistance and to secrecy, carried a common pledge of allegiance to George the Third and his successors. The recognition of the Hanoverian succession (and the repudiation of the Jacobite cause) aligned the Defenders with a campaign for Catholic emancipation directed by the Catholic Committee in Dublin. Emboldened by news of revolution and reform in France, in December 1792, the Committee called delegates, returned from all thirty-two counties on a broad parish-based franchise, to a national Catholic Convention. Representations were then made directly, over the heads of the Irish parliament and executive, to the king and his ministers in London. In the run up to the Convention, the Defenders were said, in their own "little parliaments", to have raised expectations of what Catholic representation might bring.An inquiry by the Irish House of Lords confirmed that the talk was of "being relieved of hearth money, tithes, county cesses, and of lowering their rents".

Preparing for war with the with the new French Republic, London responded to what was understood as "a threat to the whole structure of government as it had existed [in Ireland] since the 1690s", by prevailing on Dublin to match the latest measure of Catholic Relief in Britain. Catholics were readmitted, not to Parliament, but to a parliamentary franchise whose freehold threshold in the counties of a forty-shillings would exclude all but a handful of their number. A (carefully circumscribed) right to bear arms was also restored. Amid "torrents of anti-papist rhetoric", these concessions were linked to a Convention Act, which effectively outlawed any further attempt to mobilise popular opinion (with the Catholic Committee itself agreeing to disband), and to a Militia Act that gave command of both Catholic and Protestant conscripts to local gentry.

Bitter disappointment at the limits of reform and riotous opposition to conscription saw thousands taking Defender oaths. These, increasingly, bore evidence of a political radicalisation. There was reference to the Rights of Man and to revolutionary France: "The French Defenders will uphold the cause. The Irish Defenders will pull down British laws". Those taking the oaths were now as likely to be market-town traders, journeymen and artisans as cottiers and farm tenants. In Dublin, Defender lodges helped organise a broad mix of the city's working and radically disaffected population.