Which European provided the most immigrants between 1840 and 1860?

This is a very short period of time, let's think, did something relevant happened during this period, perhaps some war or famine in another part of the world to drive people out?

Yes, there was one, the Irish potato famine! so the Irish potato famine drove the Irish our of Ireland in the search for a better life, and indeed, the Irish were the biggest group to immigrate during this time!