Political - British taking over Northern Ireland.
The majority of Northern Ireland's population were unionists, who wanted to remain within the United Kingdom. (The Norman invasion of the late 12th century )
Economic - Failure of potato crop. (The Great Famine)
Religious - discrimination against the Catholic/nationalist minority by the Protestant/unionist government and police force.
Answer : Mainly because economic problems. Since potatoes were no longer available in Ireland, the Irish would starve to death since potatoes were their main resource as food.
This brings to Irish migration to the US because it offered low paying jobs for immigrants. In the end, the Irish had minimum money and enough food for themselves in America, unlike in Ireland.
One effect was selling Native Americans property.
