Analyses of the decennial U.S. federal census allows for the creation of various maps that show populations based on many different characteristics. One such type can be the number of people who live in an area based on their country of birth.
This example, Numberof Irish Foreign Born Population, 1900 and 1930, gives a broad overview of where Irish born people lived in the U.S. in those years. Unsurprisingly, the heaviest concentrations are in the northeast. One interesting concentration is the counties from Denver, CO to the border between Colorado and Kansas in 1900.