Following the start of the Great Irish Famine, two million Irish people migrated to Liverpool in the space of one decade, many of them subsequently departing for the United States. By 1851, more than 20% of the population of Liverpool was Irish. At the 2001 Census, 0.75% of the population were born in the Republic of Ireland, while 0.54% were born in Northern Ireland, but up to 50% of Liverpudlians are thought to have Irish ancestry.