Foreign Born Population of USA in 1870, Ireland 33.3%. The next is Germany at 30%.

It has been stated that in the 1870 census that the US population was around 38 million of whom almost 2 million were Irish born.

https://www.google.ie/search?q=population+us+1870&espv=2&biw=1920&bih=886&tbm=isch&imgil=e16rVItDkuHIqM%253A%253BAEWNYOW__ouXEM%253Bhttp%25253A%25252F%25252Fwww.worldmapsonline.com%25252Fwestwardmovement1870.htm&source=iu&pf=m&fir=e16rVItDkuHIqM%253A%252CAEWNYOW__ouXEM%252C_&dpr=1&usg=__wYrhq2Jyfueza4J4RpS9rcmhaY4%3D&ved=0CFgQyjdqFQoTCOKoxeHwzMcCFcpq2wodSboGEA&ei=AujgVeKfI8rV7QbJ9JqAAQ#imgrc=W4RTye5AC0PsTM%3A&usg=__wYrhq2Jyfueza4J4RpS9rcmhaY4%3D

https://www.google.ie/search?q=population+us+1870&espv=2&biw=1920&bih=886&tbm=isch&imgil=e16rVItDkuHIqM%253A%253BAEWNYOW__ouXEM%253Bhttp%25253A%25252F%25252Fwww.worldmapsonline.com%25252Fwestwardmovement1870.htm&source=iu&pf=m&fir=e16rVItDkuHIqM%253A%252CAEWNYOW__ouXEM%252C_&dpr=1&usg=__wYrhq2Jyfueza4J4RpS9rcmhaY4%3D&ved=0CFgQyjdqFQoTCOKoxeHwzMcCFcpq2wodSboGEA&ei=AujgVeKfI8rV7QbJ9JqAAQ#tbm=isch&tbs=rimg%3ACRNEODR_1spDQIjjGdW_1Ri11uBTdMTQTzSaVVZqY7JcArS08dBIKjG5o9vQiudD0F6Jli20yJRyIwlEaT8vGvY_1HQ2yoSCcZ1b9GLXW4FEWHyDIa8JsHaKhIJN0xNBPNJpVURCSD3-JIUl-QqEglmpjslwCtLTxFeROft8BYQWioSCR0EgqMbmj29EZt03foS5lGJKhIJCK50PQXomWIREKJCdZHn4MMqEgnbTIlHIjCURhFRq4d0kfQ7cSoSCZPy8a9j8dDbEb4DMFDwGMKu&q=population%20us%201870&imgrc=E0Q4NH-ykNDFEM%3A