What was the most common reason why a large number of Irish immigrants came to the untied states in the 1840's

In the early 40's, the Ireland suffered from a big famine, when 2 millions of people died. There's also politicals reasons : indeed, the Irish people was in trouble with the english one's. At least, they were also attracted by the gold rush, and a big part of them tried there selfs in USA.

So you can see that there are many reasons why the Irish people immigrated to USA.