Hundreds of thousands died in the Irish famine .
Millions more emigrated to England , Wales , Scotland , Canada , Australia , Europe , and the United States , reducing the population from about 8.5 million in 1845 to 4.3 million by 1921 .
Hundreds of thousands died in the Irish famine and millions more emigrated to England , Wales , Scotland , Canada , Australia , Europe , and the United States , reducing the population from about 8.5 million in 1845 to 4.3 million by 1921 .