The earliest record I have of Patrick O'Callahan (b ~ 1815, parents Denis and Ellen per death certificate) and Margaret Sweeney O'Callahan (b ~ 1830) is the birth of a daughter, Johanna, in 1849 in New York (inferred from later census). I don't know from what part of Ireland they came or when, or whether they came as a couple. They had a son, Eugene, born in Massachusetts in 1853, then moved to Lemonweir, Juneau County, Wisconsin, about 1855. Sons John, Joseph, William and Dennis were born there, and the surname changed to Callahan. They moved on to Waseca County, Minnesota, about 1865. Patrick died in a farm accident in 1881, and Margaret died in 1908.
Most of the children remained in Minnesota, but Joseph moved to Helena, Lewis and Clark County, Montana, in 1890 with his new bride, Augusta Goetzenberger, also fom Waseca County. He had a career as a lawman there and continued that when the family moved to Butte, Silver Bow County, in 1916. The couple had six children: Florence, Helena, William, Hazel, Joseph, and Edwin.