Planters' covenant

The Anglo-Norman invasions sponsored by King Henry II in the 12th century began the subjugation of Ireland, but English gains were massively diluted over the next few centuries. In some areas lands reverted to the control of Gaelic chieftains. In others, the now Hiberno-Norman lords intermarried with local noble families, grew independent of the Crown, spoke Irish, and often converted to Catholicism. This tide began to be reversed in 1541, when the Irish Parliament made Henry VIII King of England and Ireland, if only as a nominal overlord to the layers of local lords. But rebellions regularly disturbed the brief reign of Queen Mary I and the long one of Elizabeth.

One English solution to the problem was the plantation system, whereby colonists were imported from England to settle on land that had been confiscated from Catholics—bringing with them Protestant religion, English language, and loyalty to the Crown. (The settlers were sometimes called "New English," to distinguish them from the Old English who had gone native.) Gifford notes that "A planter who received a grant of forfeited lands was required to 'covenant' his loyalty to the English Crown by acknowledging the English sovereign as head not only of the State but also of the Church."

The plantation system continued to expand in the first half of the 17th century, under the reigns of James I, Charles I, and Oliver Cromwell. During the reign of the Scottish James, not only New English but also settlers from the Scottish Lowlands poured onto confiscated lands in Ulster, ruthlessly transforming the most Gaelic part of Ireland into the most militantly Protestant. Of these 17th century developments, Gifford notes that the system worked "so effectively that the Roman Catholic population of Ireland was virtually reduced to feudal peasantry."