Between 1845 and 1852, about 1 million people in Ireland starved to death, and 1.6 million Irish emigrated to the United States. This time in Irish history is referred to as "The Great Starvation". (Known too many as "The Great Potato Famine".)
Many farmers in Ireland were at a "poor" status before the Great Starvation. Much of the land in Ireland was broken up into two parts. One for cash crops which usually utilized the best soils, and one for subsistence crops on the poorer land. However, as marginal land was improved, subsistence croplands were often converted to cash tillage in order to pay rising rents. Thus, landlord pressure for more rents led to subsistence crops being pushed on to the least desirable land. This left many farmers poor because they couldn't crow their crops on land that they had.
Absentee landlords, most of them English, hired agents to manage their Irish farms and estates.
English landlords employing Irish-born agents found that the native men served as a middle-man against criticism. If an Irishman exploited another Irishman, who could blame the absentee landowner? Poorer and poorer land was brought under cultivation for subsistence farming.
Tithes (meaning a tenth) are levies collected in support of a church, which could be a single church or all churches of one faith. In Ireland from the 1500s to the 1800s, tithes were taxes on the agricultural system to support the Church of Ireland. By 1833, more than half the tithes due in 22 counties had not been paid. Many landowners supported non-payment because legislation of 1823 restored pastureland to the calculation. The resistance became violent, and some deaths occurred among protestors and police. Faced with an impossible situation, the authorities stopped trying to enforce payment and clergymen without income could apply for relief.