Alternative legal systems began to be used by Irish nationalist organizations during the 1760s as a means of opposing British rule in Ireland. Groups which enforced different laws included the Whiteboys, Repeal Association, Ribbonmen, Irish National Land League, Irish National League, United Irish League, Sinn Féin, and the Irish Republic during the Irish War of Independence. These alternative justice systems were connected to the agrarian protest movements which sponsored them and filled the gap left by the official authority, which never had the popular support or legitimacy which it needed to govern effectively. Opponents of British rule in Ireland sought to create an alternative system, based on Irish (rather than English) law, which would eventually supplant British authority.

British law, a chief means of enforcing British rule in Ireland, was viewed as a foreign imposition rather than a legitimate authority. From the Anglo-Norman invasion to the beginning of the seventeenth century, common law coexisted with the indigenous Brehon law. The former predominated in English-controlled areas, and the latter in other regions; in some places, both systems coexisted. The law was written and court proceedings were held in English, at a time when Irish was the sole language of most Irish people. During the sixteenth century, the surrender and regrant system was intended to co-opt Gaelic chieftains and replace Gaelic customs with English property law. The Penal Laws restricted the civil rights of Catholics until they were repealed during the 1830s. British land law enforced the property rights of landowners, ignoring Irish customs such as tenant-right. The magistrates' courts were run by unpaid landlords and other members of the Protestant Ascendancy, rather than salaried civil servants. Trust in the judicial system was further eroded by the wrongful conviction and execution of Maolra Seoighe, a monolingual Irish speaker who could not understand the court proceedings, for the 1882 Maamtrasna murders. The British government never had the support or legitimacy it needed to effectively govern Ireland, which led to the emergence of alternative systems to fill that gap.

Before the conquest the Irish people knew nothing of absolute property in land. The land virtually belonged to the entire sept, the chief was little more than the managing member of the association. The feudal idea, which views all rights as emanating from a head landlord, came in with the conquest, was associated with foreign dominion, and has never to this day been recognized by the moral sentiments of the people ... In the moral feelings of the Irish people, the right to hold the land goes, as it did in the beginning, with the right to till it.

The "unwritten law" or "unwritten agrarian code" was a deep-rooted idea among Irish smallholders that access to land for subsistence farming was a human right which superseded property rights and, regardless of titular ownership, the right to use land was hereditary and not based on the ability to pay rent. This concept had parallels in Brehon law, which did not recognize absolute property rights. Even a lord's demesne technically belonged to his entire sept. It was based on the idea that the land of Ireland rightfully belonged to the Irish people, but had been stolen by English invaders who claimed it by the right of conquest. Therefore, Irish tenants viewed the landlord–tenant relationship as inherently illegitimate and sought to abolish it. In the code's early version, practiced by the Whiteboys secret society beginning in the 1760s, it had a reactionary character which looked back to an era when there had supposedly been a reciprocal relationship between landlords and tenants. Later versions were friendly to capitalism, advocating a market economy in land and agricultural products without the "alien" landlord class.

The idea of "unwritten law" was expressed and refined by the Young Ireland activist James Fintan Lalor (1807–1849), who insisted that the Irish people had allodial title to their own land. Lalor believed that a farmer had the first right to his crop for subsistence and seed, and only then could other claims be made on the harvest. Instead of landlords evicting tenants, Lalor preferred that the landlords—"strangers here and strangers everywhere, owning no country and owned by none"—be served with a writ of ejectment. Lalor advised the Irish people to refuse "obedience to usurped authority" and resist English law, instead setting up their own government and "refus[ing] ALL rent to the present usurping proprietors". Lalor's writings were the basis of the agrarian code enforced by the Irish National Land League during the Land War in the 1880s. The tenets of the unwritten law appeared in "speeches, resolutions, placards, boycotts ... threatening letters and acts of outrage".

The Whiteboys were oath-bound secret societies in rural Ireland since the 1760s. The Threshers originated in County Mayo early in the nineteenth century and emphasized economic issues; its code regulated prices (including the price of potatoes), and demanded the reduction of the Church of Ireland's tithes and the Catholic Church's fees. Ribbon societies were first organized by poor Catholics during the 1810s. They began in northern Ireland to combat the Protestant Orange Order, but later expanded into agrarian agitation and spread southward. The Molly Maguires, who appeared in the 1840s, were often confused with Ribbonmen. Whiteboys and Ribbonism became synonymous with agrarian violence in general, and the secret societies which practiced it. The secret societies tended to pop up during agricultural depressions, and vanish in good economic times.

According to American historian Kevin Kenny, the alternative law as understood by the rural poor is the most convincing explanation for the violence practiced by these societies. Rather than a civil war by the Irish against a supposedly alien landlord class, the violence was understood as retributive justice for violations of traditional landholding and land-use practices. The rural poor could be targets if they broke their oaths to the society or otherwise failed to act in solidarity with the unwritten law. Punishments ranged from digging up new pasture land in an effort to free it up for potato cultivation, tearing down fences on newly-enclosed areas, mutilating or killing livestock, to threats and attacks on landlords' agents and merchants judged to charge exorbitant prices. Murders occurred, but were rare.

Although these societies did not systematically enforce their version of the law via a court system, a person accused of violating the code could be tried by their local society in absentia. According to Sir Thomas Larcom, "There are in fact two codes of law in force and in antagonism—one the statute law enforced by judges and jurors, in which the people do not yet trust—the other a secret law, enforced by themselves—its agents the Ribbonmen and the bullet."