Which factor caused many Irish immigrants to come to the united states in the 19th century

European Emigration to the U.S. 1851 - 1860
Although the Irish potato blight receded in 1850, the effects of the famine continued to spur Irish emigration into the 20th century. Still facing poverty and disease, the Irish set out for America where they reunited with relatives who had fled at the height of the famine.

Both countries had to aprove annexation.