What factors led to the massive increase in immigration to the US between 1820-1860 and what impact did it have on the US?

In answer to the second part of the question, the eventual result of the huge influx of immigrants was the Civil War. The first half of the century is concidered to be the birth of the industrial age. The immigrant workforce, willing and able to work long, hard hours as they assimilated to their new country, fueled this expansion and the economic power of the country shifted. Where America had largely been an agricultural nation, with money and power in the South, it became more industrial and the economic power shifted to the North. As the Southern states lost influence they rebeled. The Civil war, as with nearly every war, was fought over economics. This shift in economics is directly tracable to a large and willing workforce of immigrants arriving steadily in the North.