 The Great Famine of the 1840s caused the deaths of one million Irish people and over a million more emigrated to escape it . By the end of the decade , half of all immigration to the United States was from Ireland . The period of civil unrest that followed until the end of the 19th century is referred to as the Land War . Mass emigration became deeply entrenched and the population continued to decline until the mid @-@ 20th century . Immediately prior to the famine the population was recorded as 8 @.@ 2 million by the 1841 census . The population has never returned to this level since . The population continued to fall until 1961 and it was not until the 2006 census that the last county of Ireland ( County <unk> ) to record a rise in population since 1841 did so . 
