Short answers, pulled from the story.
The Great Famine, also called the Great Hunger or an Gorta Mor, was a period of mass starvation and disease in Ireland from 1845 to 1852. Roughly one million people died and over a million more emigrated. Irish speakers of the time called it an Drochshaol, the bad life.
The proximate cause was the infection of potato crops by the blight Phytophthora infestans, an oomycete whose origin has been traced to the Toluca Valley in Mexico. Longer-term causes included absentee landlordism, the middleman rent system, and single-crop dependence on the potato, especially the Irish Lumper variety.
The most widely accepted estimate is that one million people died, more from disease than from starvation, with the 1851 census recording 400,720 deaths from disease and 21,770 from starvation. At least a million people emigrated long-distance between 1846 and 1851, and the population fell from about 8.5 million to 4.4 million by 1901.
Large quantities of food were exported throughout the famine, and London refused to bar exports as had been done in 1782 to 1783. The Whig government followed laissez-faire doctrine, and relief funding through workhouses depended on landlords selling food to pay rates, some of which was inevitably exported. In 1846 Ireland exported 1,826 thousand quarters of grain.
The Gregory clause, named after William H. Gregory, MP, barred anyone holding more than a quarter acre of land from receiving relief. Farmers had to surrender all their land to a landlord to qualify, which drove thousands off the land. Historian James Donnelly called it indirectly a death-dealing instrument.
Total charitable donations were about 1.5 million pounds, with 856,500 pounds from outside Ireland. The British Relief Association, founded in January 1847 by Lionel de Rothschild and Abel Smith, raised roughly 390,000 pounds. Queen Victoria gave 2,000 pounds, a group of Choctaw raised 170 dollars in 1847, and Pope Pius IX issued the encyclical Praedecessores nostros calling the Catholic world to give.